As filed with the Securities and Exchange Commission on April 24, 2012

Registration No. 333-178188

UNITED STATES

SECURITIES
AND EXCHANGE COMMISSION

Washington, DC 20549

Amendment No. 5

To

FORM S-1

REGISTRATION
STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Rib-X Pharmaceuticals, Inc.

(Exact name of registrant as specified in its charter)

Delaware

2834

06-1599437

(State or other jurisdiction of

incorporation or organization)

(Primary Standard Industrial

Classification Code Number)

(IRS Employer

Identification No.)

300 George Street, Suite 301

New Haven, Connecticut 06511

(203) 624-5606

(Address, including zip code, and telephone number, including area code, of registrants principal executive offices)

Mark Leuchtenberger

President and Chief Executive Officer

Rib-X Pharmaceuticals, Inc.

300 George Street, Suite 301

New Haven, Connecticut 06511

(203) 624-5606

(Name,
address, including zip code, and telephone number, including area code, of agent for service)

With copies to:

Jonathan L. Kravetz, Esq.

Daniel H. Follansbee, Esq.

Megan N. Gates, Esq.

John T. Rudy, Esq.

Mintz,
Levin, Cohn, Ferris,

Glovsky and Popeo, P.C.

One Financial Center

Boston, Massachusetts 02111

(617) 542-6000

Michael D. Maline, Esq.

Edward A. King, Esq.

Goodwin Procter LLP

620 Eighth Avenue

New York,
NY 10018

(212) 813-8800

Approximate date of commencement of proposed sale to the public: As soon as practicable after this Registration Statement becomes effective.

If any of the securities being registered on this Form are being offered on a delayed or continuous basis pursuant to Rule 415 under the Securities
Act, check the following box. ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box
and list the Securities Act registration statement number of the earlier effective registration statement for the same
offering. ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box
and list the Securities Act registration statement number of the earlier registration statement for the same offering. ¨

If this Form is a
post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier registration statement for the same offering. ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting
company. See the definitions of large accelerated filer, accelerated filer and smaller reporting company in Rule 12b-2 of the Exchange Act.

Large accelerated filer

¨

Accelerated filer

¨

Non-accelerated filer

x (Do not check if a
smaller reporting company)

Smaller reporting company

¨

CALCULATION OF REGISTRATION FEE

Title of each class of

securities to be registered

Amount to beRegistered (1)

Proposed MaximumOffering Price PerShare (2)

Proposed Maximum

Aggregate

Offering Price
(2)

Amount of

RegistrationFee (3)(4)

Common Stock, $0.001 par value per share

6,635,500

$14.00

$92,897,000

$10,646

(1)

Includes 5,770,000 shares of common stock plus 865,500 additional shares that the underwriters have the option to purchase.

(2)

Estimated solely for the purpose of computing the amount of the registration fee pursuant to Rule 457(o) under the Securities Act of 1933, as amended.

(3)

Calculated pursuant to Rule 457(o) based on an estimate of the proposed maximum aggregate offering price of the securities registered hereunder to be sold by the Registrant.

(4)

The Registrant previously paid a registration fee of $9,168 upon the initial filing of this registration statement on November 25, 2011 with respect to a proposed maximum
aggregate offering price of $80,000,000. The additional fee of $1,478 paid in connection with the filing of this Amendment No. 5 was calculated pursuant to Rule 457(o) by multiplying the $12,897,000 increase in the proposed maximum aggregate
offering price by the current registration fee.

The Registrant hereby amends this registration statement on such
date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of
the Securities Act of 1933 or until the registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

The information in this prospectus is not complete and may be changed. We may not sell these
securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell securities, and we are not soliciting offers to buy these securities, in any state where the offer or
sale is not permitted.

Subject to Completion,
Dated April 24, 2012

PRELIMINARY PROSPECTUS

5,770,000 Shares

Common Stock

This is the initial public offering of shares of the common stock of Rib-X Pharmaceuticals,
Inc. We are offering shares of our common stock. We anticipate the initial public offering price will be between $12.00 and $14.00 per share. We have applied to list our common stock on the NASDAQ Global Market under the symbol RIBX.

We are an emerging growth company under applicable Securities and Exchange Commission rules and will be subject to reduced public
company reporting requirements. Investing in our common stock involves risks. See Risk Factors beginning on page 12.

Neither the Securities and Exchange Commission nor any other state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any
representation to the contrary is a criminal offense.

Per Share

Total

Public offering price

$

$

Underwriting discounts and commissions

$

$

Proceeds, before expenses, to us

$

$

We have granted the underwriters the right to purchase up to 865,500 additional shares of common stock to cover
over-allotments.

Certain of our existing stockholders and their affiliated entities have indicated an interest in purchasing up to approximately $20.0
million in shares of our common stock in this offering at the initial public offering price. However, because indications of interest are not binding agreements or commitments to purchase, the underwriters could determine to sell more, less or no
shares to any of these existing stockholders and any of these existing stockholders could determine to purchase more, less or no shares in this offering.

You
should rely only on the information contained in this prospectus. We have not authorized anyone to provide you with information different from that contained in this prospectus. We are offering to sell, and seeking offers to buy, shares of common
stock only in jurisdictions where offers and sales are permitted. The information contained in this prospectus is accurate only as of the date of this prospectus, regardless of the time of delivery of this prospectus or of any sale of common stock.

Until , 2012
(25 days after the date of this prospectus), all dealers that buy, sell or trade our common stock, whether or not participating in this offering, may be required to deliver a prospectus. This delivery requirement is in addition to the obligation of
dealers to deliver a prospectus when acting as underwriters and with respect to their unsold allotments or subscriptions.

This prospectus includes
estimates, statistics and other industry and market data that we obtained from industry publications, research, surveys and studies conducted by third parties and publicly available information. Such data involves a number of assumptions and
limitations and contains projections and estimates of the future performance of the industries in which we operate that are subject to a high degree of uncertainty. This prospectus also includes data based on our own internal estimates. We caution
you not to give undue weight to such projections, assumptions and estimates.

This summary provides an overview of selected information contained elsewhere in this prospectus and does not contain all of the information you
should consider before investing in our common stock. You should carefully read this prospectus and the registration statement of which this prospectus is a part in their entirety before investing in our common stock, including the information
discussed under Risk Factors and our financial statements and related notes appearing elsewhere in this prospectus. Unless otherwise indicated herein, the terms we, our, us, or the Company
refer to Rib-X Pharmaceuticals, Inc.

Overview

We are a biopharmaceutical company developing new antibiotics to provide superior coverage, safety and convenience for the treatment of serious and life-threatening infections. Our proprietary drug discovery
platform, which is based on Nobel Prize-winning science, provides an atomic-level, three-dimensional understanding of interactions between drug candidates and their bacterial targets and enables us to systematically engineer antibiotics with
enhanced characteristics. Our most advanced product candidate, delafloxacin, is intended for use as an effective and convenient first-line therapy primarily in hospitals prior to the availability of a specific diagnosis. Unlike currently available
first-line treatments, delafloxacin has the potential to offer broad-spectrum coverage as a monotherapy for serious Gram-negative and Gram-positive bacterial infections, including for methicillin-resistant Staphylococcus aureus, or MRSA, with
both intravenous and oral formulations. Most bacteria are broadly categorized as either Gram-positive, meaning that they possess a single membrane and a thick cell wall and turn dark-blue or violet when subjected to a laboratory staining method
known as Grams method, or Gram-negative, meaning that they have two membranes with a thin cell wall and, when subjected to Grams method of staining, lose the stain or are decolorized. Delafloxacin has completed four Phase 2 clinical
trials, including a Phase 2b clinical trial for the treatment of acute bacterial skin and skin structure infections, or ABSSSI. We received results from this Phase 2b trial in December 2011 and plan to commence the first of two planned Phase 3
trials for the treatment of ABSSSI in the second half of 2012. The timing of our second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this contemplated offering. Based on our current expectations
regarding the availability of such funding and subject to the results of these two trials, we anticipate submitting a New Drug Application for delafloxacin for the treatment of ABSSSI as early as the fourth quarter of 2014 and for additional
indications thereafter. Our second product candidate, radezolid, is a next-generation, IV/oral oxazolidinone designed to be a potent antibiotic with a safety profile permitting long-term treatment of resistant infections, including those caused by
MRSA. We have completed two Phase 2 clinical trials of radezolid. We are also pursuing development of RX-04, our preclinical program partnered with Sanofi, S.A., which has produced new classes of antibiotics that attach to a location on the
bacterial ribosome to which no other approved class of antibiotics bind and are designed to combat the most difficult-to-treat, multi-drug resistant Gram-positive and Gram-negative bacteria. Because its protein building function is essential for the
life of infection-causing bacteria, the bacterial ribosome is the target of most marketed antibiotics, which work by binding to the ribosome and inhibiting its function. In addition, our pipeline includes RX-05, an antibacterial discovery program,
and RX-06, an antifungal discovery program, both of which target newly discovered binding sites within ribosomes.

We believe one of our
key competitive advantages is our focus on the three-dimensional properties of antibiotics, which is enabled by our proprietary drug discovery platform. Unlike traditional approaches to antibiotic discovery, which generally rely on random screening
of chemical libraries to identify potential compounds, our discovery team utilizes sophisticated, customized computer software to simulate and predict in three-dimensions both inter- and
intra-

molecular reactions and resulting properties of compounds including absorption, distribution, metabolism, excretion and toxicology. We combine these exclusive computational tools with our
patent-protected, atomic-level insights into the structure of the ribosome to systematically engineer novel antibiotics to avoid resistance and optimize potency, spectrum, efficacy and safety. As a result, we have created a highly efficient and
productive drug development engine based on our unique design strategy that effectively leverages structure-based drug design, preparative medicinal chemistry, ribosome biochemistry, molecular biology and pharmacology.

According to Datamonitor, in the seven major pharmaceutical markets, which consist of the United States, Japan, the United Kingdom, Germany,
France, Italy and Spain, antibiotic product sales totaled approximately $20 billion in 2009 and, within the hospital market, approximately $8 billion was generated from antibiotic sales in 2006. Staphylococcus skin and soft tissue infections
in the United States alone accounted for on average nearly 12 million physician and emergency department visits annually in the years from 2001 to 2003 according to the Centers for Disease Control, or CDC. In addition, the Infectious Disease Society
of America, or IDSA, estimated in 2004 that nearly two million infections are developed in the hospital setting annually in the United States, resulting in the deaths of 90,000 patients each year. Of these infections, 70% are caused by bacteria that
are resistant to one or more antibiotics used to treat them, including those caused by MRSA. The CDC estimated that MRSA alone caused 94,000 life-threatening infections and almost 19,000 deaths in 2005 in the United States, exceeding the number of
deaths caused by HIV/AIDS in that year. Based on data provided by GlobalData for the U.S. pharmaceutical market and the global pharmaceutical market, we estimate that the use of antibiotics to treat MRSA has increased at a compounded annual growth
rate of 18% for the years from 2005 to 2010 and is forecasted to continue growing through 2017.

The three major branded antibiotics
used for the treatment of serious infections, Zyvox (linezolid), Cubicin (daptomycin) and Tygacil (tigecycline), generated U.S. sales in 2011 of $640 million, $699 million and $148 million, respectively. In addition, there were over four million
courses of vancomycin, a generic drug used to treat serious infections caused by resistant Gram-positive bacteria like MRSA, dosed in 2009.

According to the Joint Commission, formerly the Joint Commission on Accreditation of Healthcare Organizations, hospitals are generally required to begin administering antibiotics to patients with serious infections
within six hours of presentation to the hospital, well in advance of the up to 48 hours required to diagnose the particular bacteria causing the infection. As a result, this first-line antibiotic therapy needs to offer a broad spectrum of
antibacterial coverage that includes MRSA. Because there is no single broad-spectrum antibiotic available that is safe for first-line use and also has potency against MRSA, according to Datamonitor, the current first-line standard of care for
serious infections is an antibiotic cocktail consisting of the twice-daily intravenous, or IV, administration of vancomycin for MRSA coverage, and one or more additional antibiotics to broaden the overall spectrum of coverage. The use of vancomycin,
a narrow-spectrum Gram-positive treatment, may be increasingly limited due to its risk of adverse side effects and the rise of vancomycin-resistant bacterial strains in recent years.According to Datamonitor, these limitations often require
the use of a second-line treatment, such as Cubicin or Zyvox, for MRSA and other resistant Gram-positive bacteria. However, as indicated in its prescribing information, Cubicin is only available in an IV form and requires laboratory monitoring at
least weekly for toxic side effects. Although Zyvox has an available oral form, as indicated in its prescribing information, it requires active monitoring for use beyond two weeks due to the potential for significant adverse side effects, including
bone marrow suppression, or myelosuppression, and nerve damage, or neurotoxicity. In addition, studies published in TheNew England Journal of Medicine and Antimicrobial Agents and Chemotherapy have found that Cubicin and
Zyvox have also been associated with increasing drug resistance. As indicated

in its prescribing information, Tygacil, a broad-spectrum antibiotic, is generally utilized as a third- or fourth-line antibiotic due to its greater risk of mortality as compared to the active
comparators in its clinical studies, and the high rates of vomiting and nausea.

We believe that antibiotic resistance has eroded the
efficacy and exacerbated the limitations of current treatments, creating significant unmet needs for new antibiotics that represent new treatment paradigms. In particular, these include:



the need for an effective and convenient first-line, broad-spectrum antibiotic with coverage of MRSA that can be administered as a single treatment, or
monotherapy, primarily in hospitals during the critical early period of a patients care when a specific diagnosis is not yet available;



the need for a potent antibiotic with a safety profile permitting long-term treatment of resistant infections, including MRSA;



the need for drugs that treat multi-drug resistant bacteria, which are generally the most difficult to treat; and



the ongoing need for new drugs to combat the continuing problem of drug resistance.

Our unique drug discovery approach serves as the foundation for our pipeline of clinical and earlier-stage product candidates, set forth below, that we believe can address these unmet needs for the treatment of
serious infections.

Delafloxacin. Delafloxacin is intended for use as an effective and convenient
first-line antibiotic primarily in hospitals prior to the availability of a specific diagnosis. Unlike current first-line treatments, delafloxacin has the potential to offer broad-spectrum coverage as a monotherapy, including for MRSA, with both IV
and oral formulations. In addition to strong Gram-positive potency, delafloxacin has shown excellent in vitro activity against most Gram-negative bacteria commonly found in the hospital setting. We are developing both IV and oral
formulations of delafloxacin to enable patients who begin IV treatment in the hospital setting to transition to oral dosing for home-based care, offering the potential to increase patient convenience, lower the overall cost of treatment and reduce
the length of hospital stays. We believe that these attributes, combined with delafloxacins safety profile and reduced probability of resistance, demonstrate the potential of delafloxacin to become a new standard of care for first-line
treatment of serious infections and thereby reduce the need to switch to second-line, narrow-spectrum antibiotics.

We have received
results from our Phase 2b clinical trial designed to compare the efficacy of delafloxacin for the treatment of ABSSSI, including infections caused by MRSA, to Zyvox (linezolid), with and without aztreonam, and vancomycin, with and without aztreonam.
Delafloxacin met primary and secondary efficacy endpoints evaluated to date. Of note, although this Phase 2b trial was not designed to demonstrate statistical significance, for the primary endpoint of Investigators Global Assessment of Cure,
delafloxacin demonstrated a statistically significant efficacy advantage as compared to vancomycin. Additionally, delafloxacin demonstrated numerical benefit over both Zyvox (linezolid) and vancomycin in the secondary endpoint, cessation of lesion
spread and absence or resolution of fever at 48 to 72 hours. Based on this analysis and other data, we believe delafloxacin has demonstrated a level of efficacy that strongly supports our planned initiation of a Phase 3 study of delafloxacin in the
second half of 2012.

Radezolid. Radezolid is a next-generation oxazolidinone designed to meet the need for a potent
antibiotic with both IV and oral formulations and a safety profile suitable for the treatment of serious infections, including ABSSSI and severe community-acquired bacterial pneumonia, or CABP, and those caused by MRSA, as well as long-term
treatment of underserved serious infections, such as osteomyelitis and prosthetic and joint infections. Radezolid has several attributes allowing it to overcome known oxazolidinone resistance mechanisms and has shown excellent in vitro
activity against resistant Streptococcus pneumoniae and MRSA. Unlike Zyvox and tedizolid, radezolid has also shown in vitro activity against Haemophilus influenzae, Legionella pneumophila and Moraxella catarrhalis,
and other common causes of CABP. We believe that the demonstrated broad-spectrum of coverage, potency and potential long-term safety profile of radezolid give it the potential to become the antibiotic of choice for multiple resistant infections and
for treatment in populations, such as the elderly and children, that might be vulnerable to myelosuppression caused by other oxazolidinone treatments.

RX-04 Program. Our most advanced preclinical program, the RX-04 program, is focused on using one novel binding site within the ribosome to design and develop new classes of antibiotics to treat some
of the most deadly and difficult-to-treat, multi-drug resistant Gram-positive and Gram-negative infections. We also are designing candidates through the RX-04 program to have lower potential for resistance, lower potential for toxicity and potential
for IV-to-oral dosing. Using our proprietary drug discovery platform, we have developed three novel classes of antibiotics in less than three years that bind to this ribosomal site.

In June 2011, we entered into a collaboration and license agreement with Sanofi related to our
RX-04 program. Under this agreement, Sanofi has the right to license an unlimited number of product candidates targeting this discrete binding site within the ribosome. We retain all rights pertaining to our proprietary drug discovery platform,
including all other binding sites within the ribosome and all future programs, as well as to any RX-04 compound that Sanofi does not exercise its option to develop during the three-year term of the collaboration. We have received $22.0 million
through March 31, 2012 in upfront and milestone payments under the collaboration, including the receipt of a payment of $3.0 million from Sanofi in January 2012 for the achievement of a research milestone. For each RX-04 product developed by Sanofi,
we are eligible for up to $9.0 million in potential research milestone payments, up to $27.0 million in potential development milestone payments relating to initiation of Phase 1, 2 and 3 clinical trials, up to $50.0 million in potential regulatory
milestone payments relating to approvals in various jurisdictions including the United States, the European Union and Japan, up to $100.0 million in potential commercial milestone payments, and tiered percentage royalties of up to 10% on sales
from products commercialized under the agreement, if any. We also have the right under the collaboration to co-commercialize one RX-04 product of our choosing with Sanofi in the United States. We are currently collaborating with Sanofi on ongoing
preclinical development and lead generation and, as part of a comprehensive safety assessment, we have just completed in vitro and in vivo profiling of the first cohort of leads from the RX-04 program that demonstrated strong potency
and efficacy. These results have informed the next iteration of design and optimization. We expect the results of this optimization round to inform the selection of a lead compound in 2012 for toxicology studies followed by Phase 1 studies in
humans.

Our Strategy

Our objective is to discover, develop and commercialize best-in-class and new classes of anti-infectives with superior coverage, safety and convenience to provide new standards of care for patients with serious and
life-threatening infections. The critical components of our business strategy are:



Complete the clinical development of delafloxacin. We plan to commence the first of two planned Phase 3 trials for the treatment of ABSSSI in the
second half of 2012. The timing of our second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this contemplated offering. Based on our current expectations regarding the availability of such
funding and subject to the results of these two trials, we anticipate submitting applications for marketing approval to the U.S. Food and Drug Administration and European Medicines Agency as early as the fourth quarter of 2014. We also intend to
seek approval for additional indications for delafloxacin, including CABP and cIAI.



Opportunistically seek partners for the development and commercialization of our drugcandidates outside of the United States. We
have retained worldwide rights to our drug discovery platform and all of our drug discovery and development programs other than the RX-04 program, where we maintain U.S. co-commercialization rights for one product candidate of our choosing. Outside
of the United States, we expect to seek strategic partnerships for the further development and commercialization of our product candidates, including delafloxacin and radezolid. We also intend to explore additional funded collaborations leveraging
our drug discovery platform.



Advance the development of multiple product candidates from our RX-04 program through our collaboration with Sanofi. We intend to work with Sanofi
under our collaboration agreement to identify and develop multiple RX-04 product

candidates. In addition to the development and commercial milestone payments for which we are eligible for each RX-04 product candidate, we intend to exercise our right to co-commercialize one
RX-04 product of our choosing in the United States. We expect that the product candidates that emerge from the RX-04 program will target a variety of uses, including the treatment of the most deadly and difficult-to-treat, multi-drug resistant
Gram-positive and Gram-negative pathogens.



Leverage our discovery platform to continue to expand our pipeline of anti-infective product candidates. We intend to continue to pursue active
discovery programs using our proprietary platform to identify new binding sites within the ribosome and additional product candidates with broad-spectrum efficacy and safety to combat resistance mechanisms. In particular, we intend to demonstrate
evidence of potency enabling lead identification and optimization in our RX-05 antibiotic program and our RX-06 antifungal program in 2012.



Advance the clinical development of radezolid. We have successfully completed Phase 2 studies with an oral formulation of radezolid in
uncomplicated skin and skin structure infections, or uSSSI, and in CABP. Subject to obtaining sufficient additional funding beyond the proceeds of this contemplated offering, we intend to initiate a Phase 2 study for the treatment of ABSSSI and a
Phase 1 long-term safety study in humans to demonstrate what we believe is a long-term safety advantage over Zyvox. Following these studies, we also intend to perform additional clinical trials of radezolid in ABSSSI and CABP and for indications
that require long-term treatment, such as osteomyelitis and prosthetic and joint infections, including as a result of orthopedic surgery.

Risks Relating to Our Business

We are an early-stage biopharmaceutical company, and our business and ability to execute our business strategy are subject to a number of risks of which you should be aware before you decide to buy our common
stock. In particular, you should consider the following risks, which are discussed more fully in Risk Factors:



we have never been profitable, have no products approved for commercial sale and may never achieve profitability;



we will need to obtain substantial additional funding beyond this contemplated offering to complete the development and commercialization of delafloxacin and to
continue to advance the development of radezolid and our other product candidates;



we have never conducted a Phase 3 clinical trial for any of our product candidates and cannot be certain that delafloxacin or any of our other product candidates
will receive regulatory approval for commercial sale;



we may be subject to delays in our clinical trials, which could result in increased costs and delay or limit our ability to obtain regulatory approval for our
product candidates;



because the results of earlier studies and clinical trials of our product candidates may not be predictive of future clinical trial results, our product
candidates may not have favorable results in future clinical trials, which would delay or limit their future development;



we may be unable to successfully identify, develop, license or commercialize any product candidates under our collaboration with Sanofi, or to establish other

development and commercialization collaborations for delafloxacin and radezolid, which would adversely affect our ability to realize the expected benefits of such collaborations and further
develop our product candidates;



we may be unable to maintain and protect our proprietary intellectual property assets, which could impair our drug discovery platform and commercial
opportunities; and



we have incurred significant losses since our inception resulting in an accumulated deficit of $244.3 million as of December 31, 2011 and expect to incur losses
for the foreseeable future, which, among other things, raises substantial doubt about our ability to continue as a going concern.

Corporate Information

We were incorporated in Delaware in October 2000 under the name Rib-X
Designs, Inc. and changed our name to Rib-X Pharmaceuticals, Inc. in December 2000. Our primary executive offices are located at 300 George Street, Suite 301, New Haven, CT 06511-6663, and our telephone number is (203) 624-5606. Our website
address is http://www.rib-x.com. The information contained on, or that can be accessed through, our website is not part of this prospectus.

Rib-X, Rib-X Pharmaceuticals Antibiotics in Three Dimensions, and the Rib-X Pharmaceuticals logo are trademarks or registered trademarks of Rib-X Pharmaceuticals, Inc. Other trade names,
trademarks and service marks appearing in this prospectus are the property of their respective owners. Solely for convenience, the trademarks, service marks and trade names in this prospectus are referred to without the ® and TM symbols, but such references should not be construed as any indicator that their respective owners will not
assert, to the fullest extent under applicable law, their rights thereto.

We estimate that our net proceeds from the sale of 5,770,000 shares of common stock in this offering will be approximately $66.2 million after deducting estimated offering expenses and underwriting
discounts and commissions and assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover of this prospectus. We intend to use the net proceeds of this offering to fund the estimated cost of
our first planned Phase 3 clinical trial of delafloxacin, fund ongoing research and development activities for our RX-04, RX-05 and RX-06 programs, pay scheduled principal and interest on our borrowings, and other general corporate purposes.
See Use of Proceeds.

Proposed NASDAQ Global Market symbol

RIBX

The information above is based on 9,894,597 shares of our common
stock outstanding as of March 31, 2012, and assumes and gives effect to:



the issuance of 65,333 shares of our common stock upon the conversion of all outstanding shares of our convertible preferred stock and accumulated dividends
thereon, assuming that such conversion occurs on May 8, 2012;



the issuance of 9,827,456 shares of our common stock upon the conversion of all outstanding principal and interest accrued on our senior convertible demand
promissory notes, senior subordinated convertible demand promissory notes and subordinated convertible promissory notes, which we refer to collectively as our convertible notes, assuming an initial public offering price per share of $13.00, the
mid-point of the price range set forth on the cover page of this prospectus, and that such conversion occurs on May 8, 2012;



the adoption of our restated certificate of incorporation and restated by-laws in connection with the consummation of this offering;



a 1-for-5,670.66 reverse stock split of our common stock to be effected prior to the completion of this offering; and



no exercise of the underwriters over-allotment option.

It does not include:



3,857 shares and 26 shares of our common stock issuable upon the exercise of stock options outstanding as of March 31, 2012 under our 2001 Stock Option and
Incentive Plan and our 2011 Equity Incentive Plan, respectively, at a combined weighted average exercise price of $637.96 per share;

653,826 shares of our common stock issuable upon the vesting of restricted stock units granted under our 2011 Equity Incentive Plan pursuant to our Bonus Plan in
connection with this offering, assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus;



16,346 shares of our common stock issuable upon the vesting of restricted stock units granted under our 2011 Equity Incentive Plan pursuant to our Non-Employee
Director Bonus Plan in connection with this offering, assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus;



3,672 additional shares of our common stock that were available for future issuance as of March 31, 2012 under our 2011 Equity Incentive Plan, which excludes the
156,768 shares of our common stock issuable upon the exercise of options to purchase common stock to be granted in connection with this offering under our 2011 Equity Incentive Plan pursuant to a letter agreement we entered into with our new Chief
Development Officer in April 2012, assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus, and that the conversion of our preferred stock and dividends thereon
and our convertible notes and accrued interest thereon occurs on May 8, 2012, and which further excludes 1,669,362 additional shares reserved for issuance under our 2011 Equity Incentive Plan following the approval by our board of directors and
stockholders in April 2012 of our amended 2011 Equity Incentive Plan, which reserves an aggregate of 2,500,000 shares of our common stock for issuance under our 2011 Equity Incentive Plan;



160,000 shares of our common stock that will be available for future issuance under our 2012 Employee Stock Purchase Plan; and



8,230 shares of our common stock issuable upon the exercise of warrants outstanding as of March 31, 2012 at a weighted average exercise price of $869.54 per
share.

Unless otherwise indicated, all information contained in this prospectus assumes and reflects the above.

Certain of our existing stockholders and their affiliated entities have indicated an interest in purchasing up to approximately $20.0
million in shares of our common stock in this offering at the initial public offering price. However, because indications of interest are not binding agreements or commitments to purchase, the underwriters could determine to sell more, less or no
shares to any of these existing stockholders and any of these existing stockholders could determine to purchase more, less or no shares in this offering. Any shares purchased by these existing stockholders will be subject to lock-up restrictions
described in Shares Eligible for Future Sale.

You should read this summary financial data together with our audited financial statements and the related notes thereto included elsewhere in this
prospectus and the information under Selected Financial Data and Managements Discussion and Analysis of Financial Condition and Results of Operations. We derived the statement of operations data for the years ended
December 31, 2009, 2010 and 2011, and the balance sheet data as of December 31, 2011, from our audited financial statements included elsewhere in this prospectus.

The pro forma net loss per share and weighted average shares have been calculated to give effect to the (i) issuance of shares of common stock upon conversion of all
outstanding shares of our convertible preferred stock and accumulated dividends thereon and upon conversion of all outstanding principal and accrued interest on the convertible notes payable assuming an initial public offering price per share of
$13.00, the mid-point of the range set forth on the cover page of this prospectus, (ii) settlement of the put rights upon the conversion of the convertible notes payable, (iii) conversion of the preferred stock warrants into common stock
warrants and (iv) elimination of the common stock warrant exercise price protection term, in all cases, assuming each had occurred on the later of January 1, 2011 or where applicable, the issuance date of the convertible notes payable. See
Note 2 to our financial statements included elsewhere in the prospectus.

The summary unaudited pro forma balance sheet as of December 31, 2011 has been prepared to
give effect to the (i) issuance of shares of common stock upon conversion of all outstanding shares of our convertible preferred stock and accumulated dividends thereon and upon conversion of all outstanding principal and accrued interest on
the convertible notes payable, in each case, assuming an initial public offering price per share of $13.00 , the mid-point of the range set forth on the cover page of this prospectus, (ii) settlement of the put rights upon the conversion of the
convertible notes payable, (iii) conversion of the preferred stock warrants into common stock warrants, and (iv) elimination of the common stock warrant exercise price protection term, assuming in all cases, as if each had occurred on
December 31, 2011. The summary unaudited pro forma as adjusted balance sheet as of December 31, 2011 has been prepared to give effect to the foregoing items (i) through (iv) and the sale of shares of common stock in this offering after
deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us at an assumed initial public offering price per share of $13.00, the mid-point of the range set forth on the cover page of this prospectus,
assuming in all cases, as if each had occurred on December 31, 2011. The summary unaudited pro forma and pro forma as adjusted balance sheet is for informational purposes only and does not purport to indicate balance sheet information as of any
future date.

As of December 31, 2011

Actual

Pro Forma (4)

Pro Forma asAdjusted (5)

(unaudited)

(unaudited)

(in thousands)

Balance Sheet Data: (1)

Cash and cash equivalents

$

8,019

8,019

74,515

Total assets

11,690

11,515

76,160

Convertible notes payable (2)

62,143





Accrued interest on convertible notes payable (2)

14,182





Put rights

28,223





Deferred revenue, net of current portion (3)

9,997

9,997

9,997

Convertible preferred stock

122,428





Accumulated equity (deficit)

(244,264

)

(244,264

)

(244,264

)

Total stockholders equity (deficit)

(239,297

)

(12,429

)

53,830

(1)

The balance sheet data does not reflect the impact of $15,000 we borrowed under a loan and security agreement entered into in February 2012. As a result, our cash and cash
equivalents balance as of March 31, 2012 was $14,276. The aggregate principal amount outstanding under the loan and security agreement as of March 31, 2012 was $15,000. See Note 17 to our audited financial statements included elsewhere in
this prospectus for further details regarding this loan and security agreement.

(2)

Convertible notes payable and accrued interest on convertible notes payable were current liabilities as of December 31, 2011.

(3)

Deferred revenue is related to our collaboration and license agreement with Sanofi. See Note 3 to our financial statements included elsewhere in this prospectus.

(4)

The pro forma balance sheet data in the table above gives effect to the elimination of $175 of unamortized debt issuance costs upon conversion of the convertible notes payable to
stockholders as described in the paragraph above.

(5)

The pro forma as adjusted balance sheet data in the table above gives effect to the reclassification of $1,851 of deferred initial public offering costs recognized as of December
31, 2011 that will be charged to additional paid-in capital as a reduction of proceeds received in connection with this offering. As of December 31, 2011, we had made payments totaling $237 and had accrued $1,614 related to these costs.

Investing in our common stock involves a high degree of risk. You should carefully consider the following risk factors, as well as the other information in this prospectus, including our financial statements and
related notes, before deciding whether to invest in shares of our common stock. The occurrence of any of the following adverse developments described in the following risk factors could materially and adversely harm our business, financial
condition, results of operations or prospects. In that case, the trading price of our common stock could decline, and you may lose all or part of your investment.

Risks Relating Our Financial Position and Need for Additional Capital

We have never been profitable.
Currently, we have no products approved for commercial sale, and to date we have not generated any revenue from product sales. As a result, our ability to reduce our losses and reach profitability is unproven, and we may never achieve or sustain
profitability.

We have never been profitable and do not expect to be profitable in the foreseeable future. We have incurred net
losses in each year since our inception, including net losses of $28.2 million, $26.8 million and $53.5 million for 2009, 2010 and 2011, respectively. As of December 31, 2011, we had an accumulated deficit of $244.3 million. We have
devoted most of our financial resources to research and development, including our preclinical development activities and clinical trials. We have not completed development of any product candidate and we have therefore not generated any revenues
from product sales. We expect to incur increased expenses if and as we commence Phase 3 development of delafloxacin, satisfy our obligations under our agreement with Sanofi, advance our other product candidates and expand our research and
development programs. We also expect an increase in our expenses associated with seeking regulatory approvals and preparing for commercialization of our product candidates, and adding infrastructure and personnel to support our product development
efforts and operations as a public company. As a result of the foregoing, we expect to continue to experience net losses and negative cash flows for the foreseeable future. These net losses and negative cash flows have had, and will continue to
have, an adverse effect on our stockholders equity and working capital.

Because of the numerous risks and uncertainties
associated with pharmaceutical product development, we are unable to accurately predict the timing or amount of increased expenses or when, or if, we will be able to achieve profitability. In addition, our expenses could increase if we are required
by the United States Food and Drug Administration, or FDA, to perform studies in addition to those currently expected, or if there are any delays in completing our clinical trials or the development of any of our product candidates. The amount of
future net losses will depend, in part, on the rate of future growth of our expenses and our ability to generate revenues. To date, our only source of revenue has been our collaboration and license agreement with Sanofi. Future payments from Sanofi
under this collaboration are uncertain because Sanofi may choose not to continue research or development of activities for one or more potential RX-04 product candidates under the collaboration, we may not achieve milestones under the agreement with
Sanofi, Sanofi may not exercise its option to license any RX-04 product candidates, and RX-04 product candidates may not be approved or, if they are approved, may not be accepted in the market. If we are unable to develop and commercialize one or
more of our product candidates, either alone or with collaborators, or if revenues from any such collaboration product candidate that receives marketing approval are insufficient, we will not achieve profitability. Even if we do achieve
profitability, we may not be able to sustain or increase profitability.

If we are unable to raise capital when needed, we would be forced to delay, reduce or eliminate our product
development programs.

Developing pharmaceutical products, including conducting preclinical studies and clinical trials, is
expensive. We expect to incur increased expenses as we commence Phase 3 development of delafloxacin, satisfy our obligations under our agreement with Sanofi, advance our other product candidates and expand our research and development programs.
Moreover, proceeds from this offering will not be sufficient to complete the development and commercialization of our lead product candidate, delafloxacin, or to continue the development of radezolid. For instance, to complete Phase 3 development of
delafloxacin, we estimate that our two planned ABSSSI Phase 3 studies will each cost approximately $33.0 million. Accordingly, we will need to obtain additional funding beyond the proceeds of this contemplated offering to complete the
development and commercialization of delafloxacin as well as to continue to advance the development of radezolid and our other clinical and preclinical candidates. If the FDA requires that we perform additional studies beyond those that we currently
believe will be required, our expenses would further increase beyond what we currently anticipate and the anticipated timing of any potential product approvals may be delayed. Under our collaboration and license agreement with Sanofi, we are
required to use personnel and other resources in the conduct of a joint development plan directed toward identifying and optimizing product candidates thereunder meeting mutually agreed target product profiles. We currently have no commitments or
arrangements to fund our research and development programs other than future contingent milestone or royalty payments from Sanofi, which require the successful development, regulatory approval and commercialization of one or more product candidates
thereunder and may not be received for several years. We believe that the net proceeds from this offering, together with amounts we anticipate receiving under our collaboration with Sanofi and existing cash and cash equivalents and interest thereon,
will be sufficient to fund our projected operating requirements through the first quarter of 2014.

Our future funding
requirements, both short-term and long-term, will depend on many factors, including, but not limited to:



the initiation, progress, timing, costs and results of preclinical studies and clinical trials for our product candidates and potential product candidates,
including initiation of Phase 3 development for delafloxacin;



the success of our collaboration with Sanofi and receipt of milestones and royalty payments, if any, thereunder;



the number and characteristics of product candidates that we pursue;



the outcome, timing and costs of regulatory approvals;



the amount and timing of any payments we may be required to make, or that we may receive, in connection with the licensing, filing, prosecution, defense and
enforcement of any patents or other intellectual property rights;



the costs and timing of completion of commercial-scale outsourced manufacturing activities;



the costs of establishing sales, marketing and distribution capabilities for any product candidates for which we may receive regulatory approval;



the timing, receipt and amount of any sales, or royalties on, our product candidates, if any; and



the terms and timing of any future collaborative, licensing or other arrangements that we may establish.

Unless and until we can generate a sufficient amount of revenue from our product candidates, we expect
to finance future cash needs through public or private equity offerings, debt financings or regional collaborations and licensing arrangements. Additional funds may not be available when we need them on terms that are acceptable to us, or at all. If
adequate funds are not available, we may be required to delay, reduce the scope of or eliminate one or more of our research or development programs or our commercialization efforts. To the extent that we raise additional funds by issuing equity
securities, our stockholders may experience additional dilution, and debt financing, if available, may involve restrictive covenants. To the extent that we raise additional funds through collaborations and licensing arrangements, it may be necessary
to relinquish some rights to our technologies or our product candidates or grant licenses on terms that may not be favorable to us. We may be required to access the public or private capital markets from time to time when conditions are unfavorable,
or we may seek to access them when conditions are favorable even if we do not have an immediate need for additional capital.

We have a limited
operating history and we expect a number of factors to cause our operating results to fluctuate on a quarterly and annual basis, which may make it difficult to predict our future performance.

Our operations to date have been primarily limited to developing our technology and undertaking preclinical studies and clinical trials of our
product candidates. We have not yet obtained regulatory approvals for any of our product candidates. Consequently, any predictions made about our future success or viability may not be as accurate as they could be if we had a longer operating
history or approved products on the market. Our financial condition and operating results have varied significantly in the past and are expected to continue to significantly fluctuate from quarter-to-quarter or year-to-year due to a variety of
factors, many of which are beyond our control. Factors relating to our business that may contribute to these fluctuations include:

Accordingly, the results of any quarterly or annual periods should not be relied upon as indications of future operating performance.

The timing of milestone, royalty and other payments we are required to make under our license agreements are uncertain and could adversely affect our cash flows and results of operations.

We are obligated, including pursuant to an exclusive license and supply agreement with CyDex Pharmaceuticals, Inc. (now a wholly owned subsidiary of
Ligand Pharmaceuticals Incorporated, both hereafter referred to as Ligand), an exclusive license agreement with Wakunaga Pharmaceutical Co., Ltd., or Wakunaga, and an exclusive license agreement with Yale University, to make milestone payments and
pay royalties and other fees in connection with the development and commercialization of our product candidates. The timing of our achievement of these milestones and the corresponding milestone payments is subject to factors relating to the
clinical and regulatory development and commercialization of our product candidates, which are difficult to predict and for which many are beyond our control. We may become obligated to make a milestone or other payment at a time when we do not have
sufficient funds to make such payment, which could result in the loss of required intellectual property rights to further develop or commercialize one or more of our product candidates, or at a time that would otherwise require us to use funds
needed to continue to operate our business, which could delay our clinical trials, curtail our operations, scale back our commercialization and marketing efforts or seek funds to meet these obligations on terms unfavorable to us. In addition,
disputes with a licensor regarding compliance with the requirements of our agreements could result in our making milestone, royalty or other payments when we do not believe they are due to avoid potentially expensive litigation. If we are unable to
make any payment when due or if we fail to use commercially reasonable efforts to achieve certain development and commercialization milestones within the timeframes required by these agreements, the other party may have the right to terminate the
agreement and all of our rights to develop and commercialize product candidates using the applicable technology.

Our independent registered public
accounting firm has expressed doubt about our ability to continue as a going concern.

Based on our cash balances, recurring losses,
net capital deficiency, and significant debt outstanding as of December 31, 2011 and our projected spending in 2012, which raise substantial doubt about our ability to continue as a going concern, our independent registered

public accounting firm has included an explanatory paragraph in its report on our financial statements as of and for the year ended December 31, 2011 regarding this uncertainty. We believe
that the net proceeds from this offering, together with proceeds of $15.0 million from a loan agreement entered into in February 2012 and amounts we have received and anticipate receiving under our collaboration with Sanofi and existing cash and
cash equivalents and interest thereon, will be sufficient to fund our projected operating requirements through the first quarter of 2014. However, if we are unable to continue as a going concern, we might have to liquidate our assets and the values
we receive for our assets in liquidation or dissolution could be significantly lower than the values reflected in our financial statements. Amounts due under the February 2012 loan agreement may become immediately due and payable upon the occurrence
of a material adverse change, as defined under the loan agreement. Under the terms of the loan agreement, we are subject to operational covenants, including limitations on our ability to incur liens or additional debt, pay dividends, redeem stock,
make specified investments and engage in merger, consolidation or asset sale transactions, among other restrictions. In addition, the inclusion of a going concern statement by our auditors, our lack of cash resources and our potential inability to
continue as a going concern may materially adversely affect our share price and our ability to raise new capital or to enter into critical contractual relations with third parties.

We cannot be certain that delafloxacin, radezolid, our RX-04 product candidates or any of our other product candidates will receive regulatory approval, and without regulatory approval we will not be able to
market our product candidates.

We have invested a significant portion of our efforts and financial resources in the development of
our most advanced product candidates, especially delafloxacin. Our ability to generate revenue related to product sales, if ever, will depend on the successful development and regulatory approval of these product candidates.

While it is not required, we plan to request a special protocol assessment, or SPA, for our clinical protocol for each of our planned Phase 3
clinical trials of delafloxacin for the treatment of ABSSSI from the FDA. An SPA is intended to provide assurance that if the agreed upon clinical trial protocols are followed, the clinical trial endpoints are achieved, and there is a favorable
risk-benefit profile, the data may serve as the primary basis for an efficacy claim in support of an NDA. However, SPA agreements are not a guarantee of an approval of a product candidate or any permissible claims about the product candidate. In
particular, SPAs are not binding on the FDA if previously unrecognized public health concerns arise during the performance of the clinical trial, other new scientific concerns regarding product candidate safety or efficacy arise or if the sponsoring
company fails to comply with the agreed upon clinical trial protocols. We cannot predict whether we will be able to reach agreement with the FDA on an SPA or, if we do reach agreement, whether any issues will arise during the clinical trial that
would negate that agreement. In addition, we do not know how the FDA will interpret the commitments under the agreed upon SPA and how it will interpret the data and results.

We currently have no products approved for sale and we cannot guarantee that we will ever have marketable products. The development of a product
candidate and issues relating to its approval and sale are subject to extensive regulation by the FDA in the United States and regulatory authorities in other countries, with regulations differing from country to country. We are not permitted to
market our product candidates in the United States until we receive approval of an NDA from the FDA. We have not submitted an NDA for any of our product candidates. An NDA must include extensive preclinical and clinical data and supporting

information to establish the product candidates safety and effectiveness for each desired indication. The NDA must also include significant information regarding the chemistry,
manufacturing and controls for the product. Obtaining approval of an NDA is a lengthy, expensive and uncertain process, and may not be obtained. The FDA review process typically takes years to complete and approval is never guaranteed. If we submit
an NDA to the FDA, the FDA must decide whether to accept or reject the submission for filing. We cannot be certain that any submissions will be accepted for filing and review by the FDA. Even if a product is approved, the FDA may limit the
indications for which the product may be marketed, include extensive warnings on the product labeling or require expensive and time-consuming post-approval clinical trials or reporting as conditions of approval. Foreign regulatory authorities also
have requirements for approval of drug candidates with which we must comply prior to marketing. Obtaining regulatory approval for marketing of a product candidate in one country does not ensure that we will be able to obtain regulatory approval in
other countries. In addition, delays in approvals or rejections of marketing applications in the United States or foreign countries may be based upon many factors, including regulatory requests for additional analyses, reports, data and studies,
regulatory questions regarding or different interpretations of data and results, changes in regulatory policy during the period of product development and the emergence of new information regarding our product candidates or other products. Also,
regulatory approval for any of our product candidates may be withdrawn. If delafloxacin, radezolid or any of our other product candidates do not receive regulatory approval, we may not be able to generate sufficient revenue to become profitable or
to continue our operations.

Delays in the commencement, enrollment and completion of clinical trials could result in increased costs to us and delay
or limit our ability to obtain regulatory approval for our product candidates.

Delays in the commencement, enrollment and completion
of clinical trials could increase our product development costs or limit the regulatory approval of our product candidates. We plan to commence the first of two planned Phase 3 trials of delafloxacin for the treatment of ABSSSI in the second
half of 2012. However, the timing of the second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this offering. We may be unable to initiate or complete such development on schedule, if at all. In
addition, we do not know whether any future trials or studies of our other product candidates will begin on time or will be completed on schedule, if at all. The commencement, enrollment and completion of clinical trials can be delayed for a variety
of reasons, including:



inability to obtain sufficient funds required for a clinical trial;



inability to reach agreements on acceptable terms with prospective clinical research organizations, or CROs, and trial sites, the terms of which can be subject
to extensive negotiation and may vary significantly among different CROs and trial sites;



clinical holds, other regulatory objections to commencing a clinical trial or the inability to obtain regulatory approval to commence a clinical trial in those
countries that require such approvals;



inability to identify and maintain a sufficient number of trial sites, many of which may already be engaged in other clinical trial programs, including some that
may be for the same indication as our product candidates;



inability to obtain approval from institutional review boards, or IRBs, to conduct a clinical trial at their respective sites;

difficulty recruiting and enrolling patients to participate in clinical trials for a variety of reasons, including meeting the enrollment criteria for our study
and competition from other clinical trial programs for the same indication as our product candidates; and



inability to retain enrolled patients after a clinical trial is underway.

Changes in regulatory requirements and guidance may also occur and we or any of our partners may need to amend clinical trial protocols to reflect
these changes with appropriate regulatory authorities. Amendments may require us or any of our partners to resubmit clinical trial protocols to IRBs for re-examination, which may impact the costs, timing or successful completion of a clinical trial.
In addition, a clinical trial may be suspended or terminated at any time by us, our current or future partners, the FDA or other regulatory authorities due to a number of factors, including:



failure to conduct the clinical trial in accordance with regulatory requirements or our clinical protocols;

lack of adequate funding to continue the clinical trial due to unforeseen costs or other business decisions; and



upon a breach or pursuant to the terms of any agreement with, or for any other reason by, current or future partners that have responsibility for the clinical
development of any of our product candidates, including Sanofi upon exercise of its rights to develop and commercialize any RX-04 compounds.

In addition, if we or any of our partners are required to conduct additional clinical trials or other testing of our product candidates beyond those contemplated, our ability to obtain regulatory approval of these
product candidates and generate revenue from their sales would be similarly harmed.

Clinical failure can occur at any stage of clinical development
and we have never conducted a Phase 3 trial or submitted an NDA before. The results of earlier clinical trials are not necessarily predictive of future results and any product candidate we, Sanofi or our potential future partners advance through
clinical trials may not have favorable results in later clinical trials or receive regulatory approval.

Clinical failure can occur
at any stage of our clinical development. Clinical trials may produce negative or inconclusive results, and we or our partners may decide, or regulators may require us, to conduct additional clinical or preclinical testing. In addition, data
obtained from tests are susceptible to varying interpretations, and regulators may not interpret our data as favorably as we do, which may delay, limit or prevent regulatory approval. Success in preclinical testing and early clinical trials does not
ensure that subsequent clinical trials will generate the same or similar results or otherwise provide adequate data to demonstrate the efficacy and safety of a product candidate. Frequently, product candidates that have shown promising results in
early clinical trials have suffered significant setbacks in subsequent clinical trials. In addition, the design of a clinical trial can determine whether its results will support approval of a product and flaws in the design of a clinical trial may
not become apparent until the clinical trial is well advanced. We have limited experience in designing clinical trials and may be unable to design and execute a clinical trial to support regulatory approval. Further, clinical trials of potential
products often reveal that it is not practical or feasible to continue development efforts. If delafloxacin, radezolid or our other product candidates are found to be unsafe or lack efficacy, we will not be able to obtain regulatory approval for
them and our

business would be harmed. For example, if the results of our planned Phase 3 clinical trials of delafloxacin do not achieve the primary efficacy endpoints or demonstrate expected safety, the
prospects for approval of delafloxacin would be materially and adversely affected. A number of companies in the pharmaceutical industry, including those with greater resources and experience than us, have suffered significant setbacks in
Phase 3 clinical trials, even after seeing promising results in earlier clinical trials.

In some instances, there can be
significant variability in safety and/or efficacy results between different trials of the same product candidate due to numerous factors, includingchanges in trial protocols, differences in size and type of the patient populations, adherence
tothe dosing regimen and other trial protocols and the rate of dropout among clinical trialparticipants. We do not know whether any Phase 2, Phase 3 or other clinical trials we or any of our partners may conduct will
demonstrate consistent or adequate efficacy and safety to obtain regulatory approval to market our product candidates.

Our product
candidates may have undesirable side effects which may delay or prevent marketing approval, or, if approval is received, require them to be taken off the market, require them to include safety warnings or otherwise limit their sales.

We refer to those adverse events observed in our clinical trials with an incidence rate equal to or greater than 5% of the subjects in a clinical
trial as common adverse events. The common adverse events observed in clinical trials of delafloxacin were nausea, diarrhea, vomiting, pruritus, fatigue, headache, dizziness, infusion site pain, insomnia, constipation, rhinitis and dry mouth. The
common adverse events observed in clinical trials of radezolid were nausea, diarrhea, headache, dizziness and fungal infection. Three patients receiving delafloxacin in our clinical trials have had serious adverse events that were thought by the
investigator to be possibly related to delafloxacin therapy. One patient with a previously non-disclosed recent onset seizure disorder had a further seizure on delafloxacin. One patient with a complicated medical history was hospitalized with
abdominal pain and diarrhea. A third patient had a single episode of mouth swelling and shortness of breath. Two patients receiving radezolid in our clinical trials have had serious adverse events that were thought by the investigator to be possibly
related to radezolid therapy. One patient with lung cancer had a pneumonia that did not respond to radezolid therapy. A second patient with prior peptic ulcer disease discontinued ulcer therapy prior to enrolling in a radezolid trial and had a
recurrent ulcer with perforation. Additional or unforeseen side effects from these or any of our other product candidates could arise either during clinical development or, if approved, after the approved product has been marketed. Our product
candidates are being developed for the systemic treatment of multi-drug resistant and extremely-drug resistant infections caused by Gram-positive and Gram-negative bacteria and are still in the early stages of clinical development. The range and
potential severity of possible side effects from systemic therapies is significant. The results of future clinical trials may show that our product candidates cause undesirable or unacceptable side effects, which could interrupt, delay or halt
clinical trials, and result in delay of, or failure to obtain, marketing approval from the FDA and other regulatory authorities, or result in marketing approval from the FDA and other regulatory authorities with restrictive label warnings.

If any of our product candidates receives marketing approval and we or others later identify undesirable or unacceptable side effects
caused by such products:



regulatory authorities may require the addition of labeling statements, specific warnings, a contraindication or field alerts to physicians and pharmacies;



we may be required to change the way the product is administered, conduct additional clinical trials or change the labeling of the product;

regulatory authorities may require us to take our approved product off the market;



we may be subject to litigation or product liability claims; and



our reputation may suffer.

Any of these events could prevent us, Sanofi or our potential future partners from achieving or maintaining market acceptance of the affected
product or could substantially increase commercialization costs and expenses, which in turn could delay or prevent us from generating significant revenues from the sale of our products.

Reimbursement decisions by third-party payors may have an adverse effect on pricing and market acceptance. If there is not sufficient reimbursement for our products, it is less likely that our products will be
widely used.

Market acceptance and sales of delafloxacin, radezolid, or any other product candidates that we develop will depend on
reimbursement policies and may be affected by future healthcare reform measures. Government authorities and third-party payors, such as private health insurers and health maintenance organizations, decide which drugs they will cover and establish
payment levels. We cannot be certain that reimbursement will be available for delafloxacin, radezolid, or any other product candidates that we develop. Also, we cannot be certain that reimbursement policies will not reduce the demand for, or the
price paid for, our products. If reimbursement is not available or is available on a limited basis, we may not be able to successfully commercialize delafloxacin, radezolid, or any other product candidates that we develop.

In the United States, the Medicare Prescription Drug, Improvement, and Modernization Act of 2003, also called the Medicare Modernization Act, or
MMA, changed the way Medicare covers and pays for pharmaceutical products. The legislation established Medicare Part D, which expanded Medicare coverage for outpatient prescription drug purchases by the elderly but provided authority for limiting
the number of drugs that will be covered in any therapeutic class. The MMA also introduced a new reimbursement methodology based on average sales prices for physician-administered drugs.

The United States and several foreign jurisdictions are considering, or have already enacted, a number of legislative and regulatory proposals to
change the healthcare system in ways that could affect our ability to sell our products profitably. Among policy makers and payors in the United States and elsewhere, there is significant interest in promoting changes in healthcare systems with the
stated goals of containing healthcare costs, improving quality and/or expanding access to healthcare. In the United States, the pharmaceutical industry has been a particular focus of these efforts and has been significantly affected by major
legislative initiatives. We expect to experience pricing pressures in connection with the sale of delafloxacin and radezolid and any other products that we develop, due to the trend toward managed healthcare, the increasing influence of health
maintenance organizations and additional legislative proposals.

In March 2010, the Patient Protection and Affordable Care Act, as
amended by the Health Care and Education Affordability Reconciliation Act, or collectively, ACA, became law in the U.S. The goal of ACA is to reduce the cost of health care and substantially change the way health care is financed by both
governmental and private insurers. While we cannot predict what impact on federal reimbursement policies this legislation will have in general or on our business specifically, the ACA may result in downward pressure on pharmaceutical reimbursement,

which could negatively affect market acceptance of delafloxacin, radezolid or any future product candidates. Members of the U.S. Congress and some state legislatures are seeking to overturn at
least portions of the legislation and we expect they will continue to review and assess this legislation and possibly alternative health care reform proposals. We cannot predict whether new proposals will be made or adopted, when they may be adopted
or what impact they may have on us if they are adopted.

Proposed legislation before Congress specific to antibiotics may have a material impact on
antibiotic drug development.

In the past few months, identical bills intended to encourage development of antibiotics were
introduced in the U.S. House and Senate. The Generating Antibiotic Incentives Now Act, or GAIN, would provide incentives for the development of infectious disease products to address the growing epidemic of antibiotic resistant infections. The bill
recommends that qualified infectious disease products receive both Fast Track designation and Priority Review from the FDA and an additional five year period of market protection at the end of existing exclusivity periods. While GAIN may have a
positive impact on our business if enacted, there is no guarantee that it will be enacted in its current form or that we would benefit from it for delafloxacin, radezolid, or any of our other product candidates. Furthermore, GAIN may have a
disproportionately favorable effect for our competitors products compared to delafloxacin, radezolid, and our other product candidates which will make it harder for our product candidates to compete in the antibiotic market.

If we do not obtain protection under the Hatch-Waxman Amendments and similar foreign legislation by extending the patent terms and obtaining data exclusivity
for our product candidates, our business may be materially harmed.

Depending upon the timing, duration and specifics of FDA
marketing approval of delafloxacin, radezolid, and our RX-04 and other product candidates, one or more of our U.S. patents may be eligible for limited patent term restoration under the Drug Price Competition and Patent Term Restoration Act of 1984,
referred to as the Hatch-Waxman Amendments. The Hatch-Waxman Amendments permit a patent restoration term of up to five years as compensation for patent term lost during product development and the FDA regulatory review process. However, we may not
be granted an extension because of, for example, failing to apply within applicable deadlines, failing to apply prior to expiration of relevant patents or otherwise failing to satisfy applicable requirements. Moreover, the applicable time period or
the scope of patent protection afforded could be less than we request. If we are unable to obtain patent term extension or restoration or the term of any such extension is less than we request, the period during which we will have the right to
exclusively market our product will be shortened and our competitors may obtain approval of competing products following our patent expiration, and our revenue could be reduced, possibly materially. In the event that we are unable to obtain any
patent term extensions, the issued composition of matter patents for delafloxacin, delafloxacin meglumine and radezolid are expected to expire in 2016, 2027 and 2024, respectively, and, if issued, the pending composition of matter patents for our
RX-04 compounds would be expected to expire in 2030, in each case assuming the appropriate maintenance, renewal, annuity or other governmental fees are paid.

If we market products in a manner that violates healthcare fraud and abuse laws, or if we violate government price reporting laws, we may be subject to civil or criminal penalties.

In addition to FDA restrictions on marketing of pharmaceutical products, several other types of state and federal healthcare laws commonly referred
to as fraud and abuse laws have been applied in recent years to restrict certain marketing practices in the pharmaceutical industry.

These laws include false claims and anti-kickback statutes. At such time as we market our products and our products are paid for by governmental programs, it is possible that some of our business
activities could be subject to challenge under one or more of these laws.

Federal false claims laws prohibit any person from knowingly
presenting, or causing to be presented, a false claim for payment to the federal government or knowingly making, or causing to be made, a false statement to get a false claim paid. The federal healthcare program anti-kickback statute prohibits,
among other things, knowingly and willfully offering, paying, soliciting or receiving remuneration to induce, or in return for, purchasing, leasing, ordering or arranging for the purchase, lease or order of any healthcare item or service
reimbursable under Medicare, Medicaid or other federally financed healthcare programs. This statute has been interpreted to apply to arrangements between pharmaceutical manufacturers on the one hand and prescribers, purchasers and formulary managers
on the other. Although there are several statutory exemptions and regulatory safe harbors protecting certain common activities from prosecution, the exemptions and safe harbors are drawn narrowly, and practices that involve remuneration intended to
induce prescribing, purchasing or recommending may be subject to scrutiny if they do not qualify for an exemption or safe harbor. Most states also have statutes or regulations similar to the federal anti-kickback law and federal false claims laws,
which apply to items and services reimbursed under Medicaid and other state programs, or, in several states, apply regardless of the payor. Administrative, civil and criminal sanctions may be imposed under these federal and state laws.

Over the past few years, a number of pharmaceutical and other healthcare companies have been prosecuted under these laws for a variety of
promotional and marketing activities, such as: providing free trips, free goods, sham consulting fees and grants and other monetary benefits to prescribers; reporting to pricing services inflated average wholesale prices that were then used by
federal programs to set reimbursement rates; engaging in off-label promotion; and submitting inflated best price information to the Medicaid Rebate Program to reduce liability for Medicaid rebates.

If the FDA does not approve the manufacturing facilities of any future manufacturing partners for commercial production, we may not be able to commercialize any
of our product candidates.

We do not intend to manufacture the pharmaceutical products that we plan to sell. We may not be able
to identify and reach arrangement with a contract manufacturer to manufacture delafloxacin, radezolid or any of our other product candidates. For example, we have not yet reached agreement with a third-party manufacturer for the supply of
delafloxacin for our planned Phase 3 clinical trials for the treatment of ABSSSI. Additionally, the facilities used by any contract manufacturer to manufacture delafloxacin, radezolid or any of our other product candidates must be the subject of a
satisfactory inspection before the FDA approves an NDA for the product candidate manufactured at that facility. We are completely dependent on these third-party manufacturing partners for compliance with the FDAs requirements for manufacture
of our finished products. If our manufacturers cannot successfully manufacture material that conforms to our specifications and the FDAs current good manufacturing practice requirements, our product candidates will not be approved or, if
already approved, may be subject to recalls. Reliance on third-party manufacturers entails risks to which we would not be subject if we manufactured the product candidates, including:

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the possibility that we are unable to enter into a manufacturing agreement with a third party to manufacture our product candidates;

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the possible breach of the manufacturing agreements by the third parties because of factors beyond our control; and

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the possibility of termination or nonrenewal of the agreements by the third parties before we are able to arrange for a qualified replacement third-party
manufacturer.

Any of these factors could cause the delay of approval or commercialization of our products, cause us
to incur higher costs or prevent us from commercializing our product candidates successfully. Furthermore, if any of our product candidates are approved and contract manufacturers fail to deliver the required commercial quantities of finished
product on a timely basis and at commercially reasonable prices and we are unable to find one or more replacement manufacturers capable of production at a substantially equivalent cost, in substantially equivalent volumes and quality and on a timely
basis, we would likely be unable to meet demand for our products and could lose potential revenue. It may take several years to establish an alternative source of supply for our product candidates and to have any such new source approved by the FDA.

Even if our product candidates receive regulatory approval, we may still face future development and regulatory difficulties.

Our product candidates will also be subject to ongoing regulatory requirements for the labeling, packaging, storage, advertising, promotion,
record-keeping and submission of safety and other post-market information on the drug. In addition, approved products, manufacturers and manufacturers facilities are required to comply with extensive FDA requirements, including ensuring that
quality control and manufacturing procedures conform to current Good Manufacturing Practices, or cGMPs. As such, we and our contract manufacturers are subject to continual review and periodic inspections to assess compliance with cGMPs. Accordingly,
we and others with whom we work must continue to expend time, money, and effort in all areas of regulatory compliance, including manufacturing, production, and quality control. We will also be required to report certain adverse reactions and
production problems, if any, to the FDA and to comply with certain requirements concerning advertising and promotion for our products. Promotional communications with respect to prescription drugs are subject to a variety of legal and regulatory
restrictions and must be consistent with the information in the products approved label. Accordingly, we may not promote our products for indications or uses for which they are not approved.

If a regulatory agency discovers previously unknown problems with a product, such as adverse events of unanticipated severity or frequency, or
problems with the facility where the product is manufactured, or disagrees with the promotion, marketing, or labeling of a product, it may impose restrictions on that product or us, including requiring withdrawal of the product from the market. If
our product candidates fail to comply with applicable regulatory requirements, a regulatory agency may:

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issue warning letters;

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mandate modifications to promotional materials or require us to provide corrective information to healthcare practitioners;

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require us or our partners to enter into a consent decree, which can include imposition of various fines, reimbursements for inspection costs, required due dates
for specific actions and penalties for noncompliance;

We will need to obtain FDA approval of any proposed product names, and any failure or delay associated with such
approval may adversely affect our business.

Any name we intend to use for our product candidates will require approval from the FDA
regardless of whether we have secured a formal trademark registration from the United States Patent and Trademark Office, or USPTO. The FDA typically conducts a review of proposed product names, including an evaluation of the potential for confusion
with other product names. The FDA may also object to a product name if it believes the name inappropriately implies medical claims. If the FDA objects to any of our proposed product names, we may be required to adopt an alternative name for our
product candidates. If we adopt an alternative name, we would lose the benefit of our existing trademark applications for such product candidate and may be required to expend significant additional resources in an effort to identify a suitable
product name that would qualify under applicable trademark laws, not infringe the existing rights of third parties and be acceptable to the FDA. We may be unable to build a successful brand identity for a new trademark in a timely manner or at all,
which would limit our ability to commercialize our product candidates.

Risks Relating to Our Business

Even if approved, if any of our product candidates do not achieve broad market acceptance among physicians, patients and the medical community, our revenues
generated from their sales will be limited.

The commercial success of delafloxacin, radezolid, our RX-04 product candidates and our
other product candidates will depend upon their acceptance among physicians, patients and the medical community. The degree of market acceptance of our product candidates will depend on a number of factors, including:

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limitations or warnings contained in a product candidates FDA-approved labeling;

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changes in the standard of care for the targeted indications for any of our product candidates;

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limitations in the approved clinical indications for our product candidates;

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demonstrated clinical safety and efficacy compared to other products;

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lack of significant adverse side effects;

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sales, marketing and distribution support;

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availability of reimbursement from managed care plans and other third-party payors;

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timing of market introduction and perceived effectiveness of competitive products;

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the degree of cost-effectiveness;

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availability of alternative therapies at similar or lower cost, including generics and over-the-counter products;

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the extent to which the product candidate is approved for inclusion on formularies of hospitals and managed care organizations;

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whether the product is designated under physician treatment guidelines as a first-line therapy or as a second- or third-line therapy for particular infections;

If our product candidates are approved, but do not achieve an adequate level of acceptance by
physicians, patients and the medical community, sufficient revenue may not be generated from these products, and we may not become or remain profitable. In addition, efforts to educate the medical community and third-party payors on the benefits of
our product candidates may require significant resources and may never be successful.

Bacteria might develop resistance to any of our product
candidates, which would decrease the efficacy and commercial viability of those product candidates.

Drug resistance is primarily
caused by the genetic mutation of bacteria resulting from sub-optimal exposure to antibiotics where the drug does not kill all of the bacteria. While antibiotics have been developed to treat many of the most common infections, the extent and
duration of their use worldwide has resulted in new mutated strains of bacteria resistant to current treatments. We are developing product candidates to treat patients infected with drug-resistant bacteria. If physicians, rightly or wrongly,
associate the resistance issues of other products of the same class as our product candidates, physicians might not prescribe our product candidates for treating a broad range of infections. If our product candidates are improperly dosed, bacteria
might develop resistance to those product candidates causing the efficacy of these product candidates to decline, which would negatively affect our potential to generate revenues from those product candidates.

We currently have no sales and marketing infrastructure and have no experience in marketing drug products, and if we are unable to establish an effective sales
force and marketing infrastructure, or enter into acceptable third-party sales and marketing or licensing arrangements, we may not be able to commercialize our product candidates successfully.

We plan to develop a sales and marketing infrastructure to market and sell our products in the United States. We currently do not have any sales,
distribution and marketing capabilities, the development of which will require substantial resources and will be time consuming. These costs may be incurred in advance of any approval of our product candidates. In addition, we may not be able to
hire a sales force in the United States that is sufficient in size or has adequate expertise in the medical markets that we intend to target. If we are unable to establish our sales force and marketing capability, our operating results may be
adversely affected. In addition, we plan to enter into sales and marketing or licensing arrangements with third parties for international sales of any approved products. If we are unable to enter into any such arrangements on acceptable terms, or at
all, we may be unable to market and sell our products in these markets.

We face significant competition from other biotechnology and pharmaceutical
companies and our operating results will suffer if we fail to compete effectively.

The biotechnology and pharmaceutical industries
are intensely competitive and subject to rapid and significant technological change. We have competitors both in the United States and internationally, including major multinational pharmaceutical companies, established biotechnology companies,
specialty pharmaceutical and generic drug companies and universities and other research institutions. Many of our competitors have greater financial and other resources, such as larger research and development staff and more experienced marketing
and manufacturing organizations. As a result, these companies may obtain regulatory approval more rapidly than we are able to and may be more effective in selling and marketing their products as well. Smaller or early-stage companies may also prove
to be significant competitors, particularly through collaborative arrangements with large, established companies. Our competitors may succeed in developing, acquiring or licensing on an exclusive

basis, technologies and drug products that are more effective or less costly than delafloxacin, radezolid, or any other product candidates that we are currently developing or that we may develop,
which could render our products obsolete and noncompetitive.

The competition in the market for antibiotics is intense. If approved,
our product candidates will face competition from commercially available antibiotics such as vancomycin, marketed as a generic by Abbott Laboratories and others; daptomycin, marketed by Cubist Pharmaceuticals, Inc. as Cubicin; linezolid, marketed by
Pfizer Inc. as Zyvox; ceftaroline, marketed by Forest Laboratories, Inc. as Teflaro; tigecycline, marketed as Tygacil by Pfizer; and telavancin, marketed by Theravance, Inc. and Astellas Pharma, Inc. as Vibativ. Vancomycin has been a widely used and
well known antibiotic for over 40 years and is sold in a relatively inexpensive generic IV form. Vancomycin, daptomycin, ceftaroline, tigecycline, linezolid and telavancin are all approved treatments for serious Gram-positive infections such as
ABSSSI. Additionally, daptomycin is an approved treatment for bacteremia, tigecycline is an approved treatment for cIAI and CABP, linezolid is an approved treatment for pneumonia and vancomycin is an approved treatment for both bacteremia and
pneumonia. If we are unable to obtain regulatory approval of our product candidates for some or all of the indications for which our competitors are approved, we may not be able to compete effectively with such antibiotics.

In addition, if approved, our product candidates may face additional competition from antibiotics currently in clinical development. Other
antibiotics currently in development include ceftobiprole, under development by Basilea Pharmaceutica AG and approved in Canada and Switzerland, CEM-102, under development by Cempra, Inc., dalbavancin, under development by Durata Therapeutics, Inc.,
tedizolid, under development by Trius Therapeutics, Inc., NXL-103, under development by AstraZeneca PLC, oritavancin, under development by The Medicines Company, and PTK 0796, previously under development by Paratek Pharmaceuticals, Inc. and
Novartis AG, which, if approved, would compete in the antibiotic market. In addition, our product candidates may each face competition from product candidates currently in clinical development and product candidates that could receive regulatory
approval before our product candidates in countries outside the United States and the European Union. If we are unable to demonstrate the advantages of our product candidates over competing products and product candidates, we will not be able to
successfully commercialize our product candidates and our results of operations will suffer.

Established pharmaceutical companies may
also invest heavily to accelerate discovery and development of novel compounds or to in-license novel compounds that could make delafloxacin, radezolid or any other product candidates that we develop obsolete. As a result of all of these factors,
our competitors may succeed in obtaining patent protection and/or FDA approval or discovering, developing and commercializing antibiotics before we do.

If approved, delafloxacin and radezolid will face competition from less expensive generic versions of branded antibiotics of competitors, and if we are unable
to differentiate the benefits of delafloxacin or radezolid over these less expensive alternatives, we may never generate meaningful product revenues.

Generic antibiotic therapies are typically sold at lower prices than branded antibiotics and are generally preferred by insurers and other third party payors. We anticipate that, if approved, delafloxacin and
radezolid will face increasing competition in the form of generic versions of branded products of competitors that have lost or will lose their patent exclusivity. For example, both delafloxacin and radezolid, if approved, will initially face
competition from the inexpensive generic forms of vancomycin that are currently available and, in the future, may face additional competition from generic forms of other antibiotics and from generic versions of our product

after any applicable marketing exclusivity periods expire. If we are unable to demonstrate to physicians and payors that the key differentiating features of delafloxacin and radezolid translate
to overall clinical benefit or lower cost of care, we may not be able to compete with generic antibiotics.

We may not be successful in
establishing and maintaining development and commercialization collaborations, which could adversely affect our ability to develop certain of our product candidates and our financial condition and operating results.

Developing pharmaceutical products, conducting clinical trials, obtaining regulatory approval, establishing manufacturing capabilities and marketing
approved products is expensive. For example, we have entered into a research collaboration with Sanofi with respect to product candidates developed in our RX-04 program. We plan to establish additional collaborations for development and
commercialization of product candidates and research programs, including to fund the continued development of delafloxacin and radezolid. Additionally, if delafloxacin, radezolid or any of our other product candidates receives marketing approval, we
intend to enter into sales and marketing arrangements with third parties for international sales, and to develop our own sales force in the United States. If we are unable to maintain our existing arrangements or enter into any new such arrangements
on acceptable terms, if at all, we may be unable to effectively market and sell our products in our target markets. We expect to face competition in seeking appropriate collaborators. Moreover, collaboration arrangements are complex and time
consuming to negotiate, document and implement and they may require substantial resources to maintain. We may not be successful in our efforts to establish and implement collaborations or other alternative arrangements for the development of our
product candidates. When we partner with a third party for development and commercialization of a product candidate, we can expect to relinquish some or all of the control over the future success of that product candidate to the third party. Our
collaboration partner may not devote sufficient resources to the commercialization of our product candidates or may otherwise fail in their commercialization. The terms of any collaboration or other arrangement that we establish may not be favorable
to us. In addition, any collaboration that we enter into, including our collaboration with Sanofi, may be unsuccessful in the development and commercialization of our product candidates. In some cases, we may be responsible for continuing
preclinical and initial clinical development of a partnered product candidate or research program, and the payment we receive from our collaboration partner may be insufficient to cover the cost of this development. If we are unable to reach
agreements with suitable collaborators for our product candidates, we would face increased costs, we may be forced to limit the number of our product candidates we can commercially develop or the territories in which we commercialize them and we
might fail to commercialize products or programs for which a suitable collaborator cannot be found. If we fail to achieve successful collaborations, our operating results and financial condition will be materially and adversely affected.

If we fail to develop delafloxacin and radezolid for additional indications, our commercial opportunity will be limited.

To date, we have focused primarily on the development of delafloxacin for the treatment of ABSSSI. A key element of our strategy is to pursue
clinical development of delafloxacin for other indications, including CABP and cIAI. Although we believe there is substantial commercial opportunity for the treatment of ABSSSI alone, our ability to generate and grow revenues will be highly
dependent on our ability to successfully develop and commercialize delafloxacin for the treatment of these additional indications. The development of delafloxacin for these additional indications will require substantial additional funding beyond
that needed to commercialize delafloxacin for the treatment of ABSSSI and is prone to the risks of failure

inherent in drug development and we cannot provide you any assurance that we will able to successfully advance any of these programs through the development process. Even if we receive FDA
approval to market delafloxacin for the treatment of any of these additional indications, we cannot assure you that any such additional indications will be successfully commercialized, widely accepted in the marketplace or more effective than other
commercially available alternatives. If we are unable to successfully develop and commercialize delafloxacin for these additional indications, our commercial opportunity will be limited and our business prospects will suffer.

We currently plan to develop radezolid initially for the treatment of ABSSSI. A key element of our strategy is to pursue clinical development of
radezolid for other indications, including severe CABP, and long-term treatment of serious infections, such as osteomyelitis and prosthetic and joint infections. Although we believe there is substantial commercial opportunity for the treatment of
ABSSSI alone, our ability to generate and grow revenues will be highly dependent on our ability to successfully develop and commercialize radezolid for the treatment of these additional indications. The development of radezolid for these additional
indications is prone to the risks of failure inherent in drug development and we cannot provide you any assurance that we will be able to successfully advance any of these programs through the development process. Even if we receive FDA approval to
market radezolid for the treatment of any of these additional indications, we cannot assure you that any such additional indications will be successfully commercialized, widely accepted in the marketplace or more effective than other commercially
available alternatives. If we are unable to successfully develop and commercialize radezolid for these additional indications, our commercial opportunity will be limited and our business prospects will suffer.

We depend on third-party contractors for a substantial portion of our operations and may not be able to control their work as effectively as if we performed
these functions ourselves.

We outsource substantial portions of our operations to third-party service providers, including the
conduct of preclinical studies and clinical trials, chemical synthesis, biological screening and manufacturing. Our agreements with third-party service providers and clinical research organizations are on a study-by-study basis and are typically
short-term. In all cases, we may terminate the agreements with notice and are responsible for the suppliers previously incurred costs. In addition, any contract research organization that we retain will be subject to the FDAs regulatory
requirements and similar foreign standards and we do not have control over compliance with these regulations by these providers. Consequently, if these providers do not adhere to the governing practices and standards, the development and
commercialization of our product candidates could be delayed, which could severely harm our business and financial condition.

Because
we have relied on third parties, our internal capacity to perform these functions is limited. Outsourcing these functions involves the risk that third parties may not perform to our standards, may not produce results in a timely manner or may fail
to perform at all. In addition, the use of third-party service providers requires us to disclose our proprietary information to these parties, which could increase the risk that this information will be misappropriated. There is a limited number of
third-party service providers that specialize or have the expertise required to achieve our business objectives. Identifying, qualifying and managing performance of third-party service providers can be difficult, time consuming and cause delays in
our development programs. We currently have a small number of employees, which limits the internal resources we have available to identify and monitor our third-party providers. To the extent we are unable to identify, retain and successfully manage
the performance of third-party service providers in the future, our business may be adversely affected.

We will need to expand our operations and increase the size of our company, and we may experience difficulties in
managing growth.

As we increase the number of ongoing product development programs and advance our product candidates through
preclinical studies and clinical trials, we will need to increase our product development, scientific and administrative headcount to manage these programs. In addition, to meet our obligations as a public company, we will need to increase our
general and administrative headcount. Our management, personnel and systems currently in place may not be adequate to support this future growth. Our need to effectively manage our operations, growth and various projects requires that we:

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successfully attract and recruit new employees with the expertise and experience we will require;

If we are unable to successfully manage this growth, our business may be adversely affected.

We may not be able to manage our business effectively if we are unable to attract and retain key personnel.

We may not be able to attract or retain qualified management, finance, scientific and clinical personnel in the future due to the intense
competition for qualified personnel among biotechnology, pharmaceutical and other businesses. If we are not able to attract and retain necessary personnel to accomplish our business objectives, we may experience constraints that will significantly
impede the achievement of our development objectives, our ability to raise additional capital and our ability to implement our business strategy.

Our industry has experienced a high rate of turnover of management personnel in recent years. We are highly dependent on the development, regulatory, commercialization and business development expertise of our
executive officers and key employees identified in the Management section of this prospectus. If we lose one or more of our executive officers or key employees, our ability to implement our business strategy successfully could be
seriously harmed. We intend to enter into change of control and severance agreements with each of our officers as part of our retention efforts. The terms of these agreements are described in the Executive CompensationPotential Payments
upon Termination or Change in Control section of this prospectus. However, any of our executive officers or key employees may terminate their employment at any time. Replacing executive officers and key employees may be difficult and may take
an extended period of time because of the limited number of individuals in our industry with the breadth of skills and experience required to develop, gain regulatory approval of and commercialize products successfully. Competition to hire from this
limited pool is intense, and we may be unable to hire, train, retain or motivate these additional key personnel. Our failure to retain key personnel could materially harm our business.

We are an emerging growth company and our election to delay adoption of new or revised accounting
standards applicable to public companies may result in our financial statements not being comparable to those of other public companies. As a result of this and other reduced disclosure requirements applicable to emerging growth companies, our
common stock may be less attractive to investors.

We are an emerging growth company, as defined in the Jumpstart Our
Business Startups Act of 2012, or the JOBS Act, and we intend to take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies including, but
not limited to, not being required to comply with the auditor attestation requirements of Section 404 of the Sarbanes-Oxley Act, reduced disclosure obligations regarding executive compensation in our periodic reports and proxy statements, and
exemptions from the requirements of holding a nonbinding advisory vote on executive compensation and shareholder approval of any golden parachute payments not previously approved. In addition, Section 107 of the JOBS Act also provides that an
emerging growth company can take advantage of the extended transition period provided in Section 7(a)(2)(B) of the Securities Act for complying with new or revised accounting standards. In other words, an emerging growth
company can delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. We are electing to delay such adoption of new or revised accounting standards, and as a result, we may not comply
with new or revised accounting standards on the relevant dates on which adoption of such standards is required for non-emerging growth companies. As a result of such election, our financial statements may not be comparable to the financial
statements of other public companies. We cannot predict if investors will find our common stock less attractive because we will rely on these exemptions. If some investors find our common stock less attractive as a result, there may be a less active
trading market for our common stock and our stock price may be more volatile. We may take advantage of these reporting exemptions until we are no longer an emerging growth company. We will remain an emerging growth company
for up to five years, although if the market value of our common stock that is held by non-affiliates exceeds $700 million as of any June 30 before that time, we would cease to be an emerging growth company as of the following
December 31.

As a public company, we will
operate in an increasingly demanding regulatory environment, which requires us to comply with the Sarbanes-Oxley Act of 2002, or the Sarbanes-Oxley Act, and the related rules and regulations of the Securities and Exchange Commission, expanded
disclosure requirements, accelerated reporting requirements and more complex accounting rules. Company responsibilities required by the Sarbanes-Oxley Act include establishing corporate oversight and adequate internal control over financial
reporting and disclosure controls and procedures. Effective internal controls are necessary for us to produce reliable financial reports and are important to help prevent financial fraud.

We have begun implementing our system of internal controls over financial reporting and preparing the documentation necessary to perform the
evaluation needed to comply with Section 404 of the Sarbanes-Oxley Act. Although we will need to hire additional finance personnel and build our financial infrastructure as we transition to operating as a public company, including complying with the
requirements of Section 404 of the Sarbanes-Oxley Act, we have recently taken actions to improve our financial infrastructure, including the hiring of a corporate controller and an accounting staff person. Following this offering as we begin

operating as a public company, we will continue improving our financial infrastructure with the hiring of additional financial and accounting staff, the enhancement of internal controls, and
additional training for our financial and accounting staff.

Section 404 of the Sarbanes-Oxley Act requires annual management
assessments of the effectiveness of our internal control over financial reporting, starting with the second annual report that we would expect to file with the Securities and Exchange Commission. However, for as long as we remain an emerging
growth company as defined in the JOBS Act, we intend to take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies including, but
not limited to, not being required to comply with the auditor attestation requirements of Section 404 of the Sarbanes-Oxley Act. We may take advantage of these reporting exemptions until we are no longer an emerging growth company.
We will remain an emerging growth company for up to five years, although if the market value of our common stock that is held by non-affiliates exceeds $700 million as of any June 30 before that time, we would cease to be an
emerging growth company as of the following December 31.

Until we are able to expand our finance and
administrative capabilities and establish necessary financial reporting infrastructure, we may not be able to prepare and disclose, in a timely manner, our financial statements and other required disclosures or comply with the Sarbanes-Oxley Act or
existing or new reporting requirements. If we cannot provide reliable financial reports or prevent fraud, our business and results of operations could be harmed and investors could lose confidence in our reported financial information.

Our employees may engage in misconduct or other improper activities, including noncompliance with regulatory standards and requirements and insider trading.

We are exposed to the risk of employee fraud or other misconduct. Misconduct by employees could include intentional failures to
comply with FDA regulations, provide accurate information to the FDA, comply with federal and state health care fraud and abuse laws and regulations, report financial information or data accurately or disclose unauthorized activities to us. In
particular, sales, marketing and business arrangements in the health care industry are subject to extensive laws and regulations intended to prevent fraud, misconduct, kickbacks, self-dealing and other abusive practices. These laws and regulations
may restrict or prohibit a wide range of pricing, discounting, marketing and promotion, sales commission, customer incentive programs and other business arrangements. Employee misconduct could also involve the improper use of information obtained in
the course of clinical trials, which could result in regulatory sanctions and serious harm to our reputation. We intend to adopt a code of conduct prior to the completion of this offering, but it is not always possible to identify and deter employee
misconduct, and the precautions we take to detect and prevent this activity may not be effective in controlling unknown or unmanaged risks or losses or in protecting us from governmental investigations or other actions or lawsuits stemming from a
failure to comply with these laws or regulations. If any such actions are instituted against us, and we are not successful in defending ourselves or asserting our rights, those actions could have a significant impact on our business, including the
imposition of significant fines or other sanctions.

We face potential product liability exposure, and if successful claims are brought against us, we may incur
substantial liability for a product candidate and may have to limit its commercialization.

The use of our product candidates in
clinical trials and the sale of any products for which we may obtain marketing approval expose us to the risk of product liability claims. Product liability claims may be brought against us or our partners by participants enrolled in our clinical
trials, patients, health care providers or others using, administering or selling our products. If we cannot successfully defend ourselves against any such claims, we would incur substantial liabilities. Regardless of merit or eventual outcome,
product liability claims may result in:

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withdrawal of clinical trial participants;

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termination of clinical trial sites or entire trial programs;

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costs of related litigation;

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substantial monetary awards to patients or other claimants;

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decreased demand for our product candidates and loss of revenues;

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impairment of our business reputation;

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diversion of management and scientific resources from our business operations; and

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the inability to commercialize our product candidates.

We have obtained limited product liability insurance coverage for our clinical trials domestically and in selected foreign countries where we are conducting clinical trials. Our product liability insurance coverage
is currently limited to $5 million per occurrence with an annual aggregate limit of $5 million. As such, our insurance coverage may not reimburse us or may not be sufficient to reimburse us for any expenses or losses we may suffer. Moreover,
insurance coverage is becoming increasingly expensive, and, in the future, we may not be able to maintain insurance coverage at a reasonable cost or in sufficient amounts to protect us against losses due to product liability. We intend to expand our
insurance coverage for products to include the sale of commercial products if we obtain marketing approval for our product candidates in development, but we may be unable to obtain commercially reasonable product liability insurance for any products
approved for marketing. Large judgments have been awarded in class action lawsuits based on drugs that had unanticipated side effects. A successful product liability claim or series of claims brought against us, particularly if judgments exceed our
insurance coverage, could decrease our cash resources and adversely affect our business.

Our insurance policies are expensive and protect us only
from some business risks, which will leave us exposed to significant uninsured liabilities.

We do not carry insurance for all
categories of risk that our business may encounter. For example, we do not carry earthquake insurance. In the event of a major earthquake in our region, our business could suffer significant and uninsured damage and loss. Some of the policies we
currently maintain include general liability, employment practices liability, property, auto, workers compensation, products liability and directors and officers insurance. We do not know, however, if we will be able to maintain
existing insurance with adequate levels of coverage. Any significant uninsured liability may require us to pay substantial amounts, which would adversely affect our cash position and results of operations.

Our operations involve hazardous materials, which could subject us to significant liabilities.

Our research and development processes involve the controlled use of hazardous materials, including chemicals. Our operations produce hazardous
waste products. We cannot eliminate the risk of accidental contamination or discharge or injury from these materials. Federal, state and local laws and regulations govern the use, manufacture, storage, handling and disposal of these materials. We
could be subject to civil damages in the event of exposure of individuals to hazardous materials. In addition, claimants may sue us for injury or contamination that results from our use of these materials and our liability may exceed our total
assets. We have general liability insurance of up to $2 million per occurrence with an annual aggregate limit of $2 million, which excludes pollution liability. We also have umbrella liability insurance of up to $5 million per occurrence with an
annual aggregate limit of $5 million, which excludes product liability. This coverage may not be adequate to cover all claims related to our biological or hazardous materials. Furthermore, if we were to be held liable for a claim involving our
biological or hazardous materials, this liability could exceed our insurance coverage, if any, and our other financial resources. Compliance with environmental and other laws and regulations may be expensive and current or future regulations may
impair our research, development or production efforts.

Risks Relating to Our Intellectual Property

Our ability to pursue the development and commercialization of delafloxacin depends upon the continuation of our license from Wakunaga.

Our license agreement with Wakunaga provides us with a worldwide exclusive license to develop and sell delafloxacin. In particular, we obtained an
exclusive license to certain patents, patent applications and proprietary information covering the composition of matter to delafloxacin, and rights to other patents and applications, which license requires us to make certain payments to Wakunaga.
If we are unable to make the required milestone and royalty payments under the license agreement, or if we do not use commercially reasonable efforts to achieve certain development and commercialization milestones for delafloxacin within the
timeframes required by the license agreement, our rights to develop and commercialize delafloxacin could be terminated and would revert to Wakunaga. In addition, either we or Wakunaga may terminate the license agreement upon a material breach of the
license agreement not cured within 90 days from notice of breach. If our license agreement with Wakunaga were terminated, we would lose our rights to develop and commercialize delafloxacin, which would materially and adversely affect our business,
results of operations and future prospects.

It is difficult and costly to protect our proprietary rights, and we may not be able to ensure their
protection. If our patent position does not adequately protect our product candidates, others could compete against us more directly, which would harm our business, possibly materially.

Our commercial success will depend in part on obtaining and maintaining patent protection and trade secret protection of our ribosome-based
discovery platform and of our current and future product candidates and the methods used to manufacture them, as well as successfully defending these patents against third-party challenges. Our ability to stop third parties from making, using,
selling, offering to sell or importing our products and ribosome-based discovery platform is dependent upon the extent to which we have rights under valid and enforceable patents or trade secrets that cover these activities.

The patent positions of pharmaceutical companies can be highly uncertain and involve complex legal and
factual questions for which important legal principles remain unresolved. No consistent policy regarding the breadth of claims allowed in pharmaceutical patents has emerged to date in the United States or in many foreign jurisdictions. Changes in
either the patent laws or interpretations of patent laws in the United States and foreign jurisdictions may diminish the value of our intellectual property. Accordingly, we cannot predict the breadth of claims that may be enforced in the patents
that may be issued from the applications we currently or may in the future own or license from third parties. Further, if any patents we obtain or license are deemed invalid and unenforceable, our ability to commercialize or license our technology
could be adversely affected.

The degree of future protection for our proprietary rights is uncertain because legal means afford only
limited protection and may not adequately protect our rights or permit us to gain or keep our competitive advantage. For example:

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others may be able to develop a platform similar to, or better than, ours in a way that is not covered by the claims of our patents;

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others may be able to make compounds that are similar to our product candidates but that are not covered by the claims of our patents;

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we might not have been the first to make the inventions covered by our pending patent applications;

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we might not have been the first to file patent applications for these inventions;

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others may independently develop similar or alternative technologies or duplicate any of our technologies;

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any patents that we obtain may not provide us with any competitive advantages;



we may not develop additional proprietary technologies that are patentable; or

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the patents of others may have an adverse effect on our business.

As of March 31, 2012, we were the owner of record of over 15 issued or granted U.S. and foreign patents with claims directed to pharmaceutical compounds, pharmaceutical compositions, methods of making these
compounds, and methods of using these compounds in various indications, and also to ribosome-based technology platforms and drug discovery methods. We were also the owner of record of over 150 pending U.S. and foreign patent applications in these
areas.

As of March 31, 2012, we were the licensee of over 40 issued or granted U.S. and foreign patents and over 20 pending U.S. and
foreign patent applications, with claims directed to pharmaceutical compounds, pharmaceutical compositions, methods of making these compounds, methods of using these compounds in various indications, and also to ribosome-based technology platforms
and drug discovery methods.

Due to the patent laws of a country, or the decisions of a patent examiner in a country, or our own
filing strategies, we may not obtain patent coverage for all of our product candidates or methods involving these candidates or for our ribosome-based technology platform in the parent patent application. We plan to pursue divisional patent
applications or continuation patent applications in the United States and many other countries to obtain claim coverage for inventions which were disclosed but not claimed in the parent patent application.

We may also rely on trade secrets to protect our technology, especially where we do not believe patent protection is appropriate or feasible.
However, trade secrets are difficult to

protect. Although we use reasonable efforts to protect our trade secrets, our employees, consultants, contractors, outside scientific collaborators and other advisors may unintentionally or
willfully disclose our information to competitors. Enforcing a claim that a third party illegally obtained and is using any of our trade secrets is expensive and time consuming, and the outcome is unpredictable. In addition, courts outside the
United States are sometimes less willing to protect trade secrets. Moreover, our competitors may independently develop equivalent knowledge, methods and know-how.

We may incur substantial costs as a result of litigation or other proceedings relating to patent and other intellectual property rights.

If we choose to go to court to stop another party from using the inventions claimed in any patents we obtain, that individual or company has the
right to ask the court to rule that such patents are invalid or should not be enforced against that third party. These lawsuits are expensive and would consume time and resources and divert the attention of managerial and scientific personnel even
if we were successful in stopping the infringement of such patents. In addition, there is a risk that the court will decide that such patents are not valid and that we do not have the right to stop the other party from using the inventions. There is
also the risk that, even if the validity of such patents is upheld, the court will refuse to stop the other party on the ground that such other partys activities do not infringe our rights to such patents. In addition, the U.S. Supreme Court
has recently modified some tests used by the USPTO in granting patents over the past 20 years, which may decrease the likelihood that we will be able to obtain patents and increase the likelihood of challenge of any patents we obtain or
license.

We may infringe the intellectual property rights of others, which may prevent or delay our product development efforts and stop us from
commercializing or increase the costs of commercializing our products.

Our success will depend in part on our ability to operate
without infringing the proprietary rights of third parties. Patents of which we are not aware, and that our products infringe, may be issued. Additionally, patents that we believe we do not infringe, but that we may ultimately be found to infringe,
could be issued. Furthermore, a third party may claim that we or our manufacturing or commercialization partners are using inventions covered by the third partys patent rights and may go to court to stop us from engaging in our normal
operations and activities, including making or selling our product candidates. These lawsuits are costly and could affect our results of operations and divert the attention of managerial and scientific personnel. There is a risk that a court would
decide that we or our commercialization partners are infringing the third partys patents and would order us or our partners to stop the activities covered by the patents. In that event, we or our commercialization partners may not have a
viable way around the patent and may need to halt commercialization of the relevant product with it. In addition, there is a risk that a court will order us or our partners to pay the other party damages for having violated the other partys
patents. In the future, we may agree to indemnify our commercial partners against certain intellectual property infringement claims brought by third parties. The pharmaceutical and biotechnology industries have produced a proliferation of patents,
and it is not always clear to industry participants, including us, which patents cover various types of products or methods of use. The coverage of patents is subject to interpretation by the courts, and the interpretation is not always uniform. If
we are sued for patent infringement, we would need to demonstrate that our products or methods either do not infringe the patent claims of the relevant patent or that the patent claims are invalid, and we may not be able to do this. Proving
invalidity is difficult. For example, in the United States, proving invalidity requires a showing of clear and convincing evidence to overcome the presumption of validity enjoyed by issued patents. Even if we are successful in these

proceedings, we may incur substantial costs and divert managements time and attention in pursuing these proceedings, which could have a material adverse effect on us. If we are unable to
avoid infringing the patent rights of others, we may be required to seek a license, defend an infringement action or challenge the validity of the patents in court. Patent litigation is costly and time consuming. We may not have sufficient resources
to bring these actions to a successful conclusion. In addition, if we do not obtain a license, develop or obtain non-infringing technology, fail to defend an infringement action successfully or have infringed patents declared invalid, we may incur
substantial monetary damages, encounter significant delays in bringing our product candidates to market and be precluded from manufacturing or selling our product candidates.

Because some patent applications in the United States may be maintained in secrecy until the patents are issued, because patent applications in the United States and many foreign jurisdictions are typically not
published until eighteen months after the priority date, and because publications in the scientific literature often lag behind actual discoveries, we cannot be certain that others have not filed patent applications for technology covered by our
pending applications, or that we were the first to invent the technology. Our competitors may have filed, and may in the future file, patent applications covering technology similar to ours. Any such patent application may have priority over our
patent applications, which could further require us to obtain rights to issued patents covering such technologies. If another party has filed a U.S. patent application on inventions similar to ours, we may have to participate in an interference
proceeding declared by the USPTO to determine priority of invention in the United States. The costs of these proceedings could be substantial, and it is possible that such efforts would be unsuccessful if, unbeknownst to us, the other party had
independently arrived at the same or similar invention prior to our own invention, resulting in a loss of our U.S. patent position with respect to such inventions.

Patents covering the composition of matter of delafloxacin expire in 2016, excluding any additional term for patent term adjustments or patent term extensions. We expect that the other patents and patent
applications in the delafloxacin portfolio, if issued, and if the appropriate maintenance, renewal, annuity or other governmental fees are paid, would expire between 2025 and 2029. Patents covering the composition of matter of radezolid expire in
2024, excluding any additional term for patent term adjustments or patent term extensions. We expect the other patents and patent applications in the radezolid portfolio, if issued, and if the appropriate maintenance, renewal, annuity or other
governmental fees are paid, to expire between 2024 and 2031. Our patent applications and patents include or support claims on other aspects of delafloxacin and radezolid such as pharmaceutical formulations containing delafloxacin and radezolid,
methods of using delafloxacin and radezolid to treat disease and methods of manufacturing delafloxacin and radezolid. Without patent protection on the composition of matter of delafloxacin or radezolid, our ability to assert our patents to stop
others from using or selling delafloxacin or radezolid in a non-pharmaceutically acceptable formulation may be limited.

Some of our
competitors may be able to sustain the costs of complex patent litigation more effectively than we can because they have substantially greater resources. In addition, any uncertainties resulting from the initiation and continuation of any litigation
could have a material adverse effect on our ability to raise the funds necessary to continue our operations.

Obtaining and maintaining our patent protection depends on compliance with various procedural, document submission,
fee payment and other requirements imposed by governmental patent agencies, and our patent protection could be reduced or eliminated for non-compliance with these requirements.

Periodic maintenance fees, renewal fees, annuity fees and various other governmental fees on patents and/or applications will be due to be paid to
the USPTO and various foreign governmental patent agencies in several stages over the lifetime of the patents and/or applications. We have systems in place to remind us to pay these fees, and we employ an outside firm, CPA Global Limited, and rely
on our outside counsel and Yale University, to pay these fees due to foreign patent agencies. The USPTO and various foreign governmental patent agencies require compliance with a number of procedural, documentary, fee payment and other similar
provisions during the patent application process. We employ reputable law firms and other professionals to help us comply, and in many cases, an inadvertent lapse can be cured by payment of a late fee or by other means in accordance with the
applicable rules. However, there are situations in which noncompliance can result in abandonment or lapse of the patent or patent application, resulting in partial or complete loss of patent rights in the relevant jurisdiction. In such an event, our
competitors might be able to enter the market and this circumstance would have a material adverse effect on our business.

We have not yet registered
our trademarks in all of our potential markets, and failure to secure those registrations could adversely affect our business.

We
have filed trademark applications with the USPTO for our marks, Rib-X and Rib-X Pharmaceuticals Antibiotics in Three Dimensions for use in connection with our services and anticipate also filing with respect to these marks at
the appropriate time in conjunction with our goods. We also anticipate filing foreign trademark applications for the same marks for goods and services outside the United States. The Rib-X mark has been approved for publication by the
USPTO, but is subject to a 30-day public opposition period, which can be extended by an additional 90 days upon the request of an interested party. It is possible that the marks could be opposed or cancelled after registration. The registrations
will be subject to use and maintenance requirements. We have not yet registered all of our trademarks in all of our potential markets, and it is also possible that there are names or symbols other than Rib-X and Rib-X
Pharmaceuticals Antibiotics in Three Dimensions that may be protectable marks for which we have not sought registration, and failure to secure those registrations could adversely affect our business. We cannot assure you that opposition or
cancellation proceedings will not be filed against our trademarks or that our trademarks would survive such proceedings.

We have not
yet registered trademarks for any of our product candidates in any jurisdiction. When we file trademark applications for our product candidates in the U.S., our trademark applications in the U.S. and any other jurisdictions where we may file may not
be allowed for registration, and registered trademarks may not be obtained, maintained or enforced. During trademark registration proceedings, we may receive rejections. Although we are given an opportunity to respond to those rejections, we may be
unable to overcome such rejections. In addition, in the USPTO and in comparable agencies in many foreign jurisdictions, third parties are given an opportunity to oppose pending trademark applications and to seek to cancel registered trademarks.
Opposition or cancellation proceedings may be filed against our trademarks, and our trademarks may not survive such proceedings.

We may be subject to claims that our employees have wrongfully used or disclosed alleged trade secrets of their
former employers. If we are not able to adequately prevent disclosure of trade secrets and other proprietary information, the value of our technology and products could be significantly diminished.

As is common in the biotechnology and pharmaceutical industries, we employ individuals who were previously employed at other biotechnology or
pharmaceutical companies, including our competitors or potential competitors. We may be subject to claims that these employees, or we, have inadvertently or otherwise used or disclosed trade secrets or other proprietary information of their former
employers. Litigation may be necessary to defend against these claims. Even if we are successful in defending against these claims, litigation could result in substantial costs and be a distraction to management.

We rely on trade secrets to protect our proprietary technologies, especially where we do not believe patent protection is appropriate or
obtainable. However, trade secrets are difficult to protect. We rely in part on confidentiality agreements with our employees, consultants, outside scientific collaborators, sponsored researchers and other advisors to protect our trade secrets and
other proprietary information. These agreements may not effectively prevent disclosure of confidential information and may not provide an adequate remedy in the event of unauthorized disclosure of confidential information. In addition, others may
independently discover our trade secrets and proprietary information. For example, the FDA, as part of its Transparency Initiative, is currently considering whether to make additional information publicly available on a routine basis, including
information that we may consider to be trade secrets or other proprietary information, and it is not clear at the present time how the FDAs disclosure policies may change in the future, if at all. Costly and time-consuming litigation could be
necessary to enforce and determine the scope of our proprietary rights, and failure to obtain or maintain trade secret protection could adversely affect our competitive business position.

Risks Relating to Owning Our Common Stock

No public market for our common stock currently exists and an active trading market may not develop or be sustained following this offering.

Prior to this offering, there has been no public market for our common stock. An active trading market may not develop following the completion of
this offering or, if developed, may not be sustained. Certain of our existing stockholders and their affiliated entities have indicated an interest in purchasing up to approximately $20.0 million in shares of our common stock in this offering at the
initial public offering price. To the extent these existing stockholders are allocated and purchase shares in this offering, such purchases would reduce the available public float for our shares because these existing stockholders will be restricted
from selling the shares under the lock-up agreements described in the Shares Eligible for Future Sale section of this prospectus. As a result, the liquidity of our common stock could be significantly reduced from what it would have been
if these shares had been purchased by investors that were not affiliated with us. The lack of an active market may impair your ability to sell your shares at the time you wish to sell them or at a price that you consider reasonable. The lack of an
active market may also reduce the fair market value of your shares. An inactive market may also impair our ability to raise capital to continue to fund operations by selling shares and may impair our ability to acquire other companies or
technologies by using our shares as consideration.

Our share price may be volatile, which could subject us to securities class action litigation and prevent you from
being able to sell your shares at or above the offering price.

The initial public offering price for our shares will be determined
by negotiations between us and the representative of the underwriters and may not be indicative of prices that will prevail in the trading market. The market price of shares of our common stock could be subject to wide fluctuations in response to
many risk factors listed in this section, and others beyond our control, including:

disputes or other developments related to proprietary rights, including patents, litigation matters and our ability to obtain patent protection for our
technologies;

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announcement or expectation of additional financing efforts;

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sales of our common stock by us, our insiders or our other stockholders;



market conditions for biopharmaceutical stocks in general; and



general economic and market conditions.

Furthermore, the stock markets have experienced extreme price and volume fluctuations that have affected and continue to affect the market prices of equity securities of many companies. These fluctuations often
have been unrelated or disproportionate to the operating performance of those companies. These broad market and industry fluctuations, as well as general economic, political and market conditions such as recessions, interest rate changes or
international currency fluctuations, may negatively impact the market price of shares of our common stock. In addition, such fluctuations could subject us to securities class action litigation, which could result in substantial costs and divert our
managements attention from other business concerns, which could seriously harm our business. If the market price of shares of our common stock after this offering does not exceed the initial public offering price, you may not realize any
return on your investment in us and may lose some or all of your investment.

We have a significant stockholder, which will limit your ability to influence corporate matters and may give
rise to conflicts of interest.

When this offering is completed, affiliates of Warburg Pincus LLC, or Warburg Pincus, are expected to
beneficially own shares representing approximately 44.4% of our common stock, assuming that the closing of the offering made hereby occurs with an initial public offering price per share of $13.00, the mid-point of the price range set forth on the
cover page of this prospectus, and the conversion of our preferred stock and accumulated dividends thereon and convertible notes and accrued interest thereon into common stock occurs on May 8, 2012. Accordingly, Warburg Pincus will exert significant
influence over us and any action requiring the approval of the holders of our common stock, including the election of directors and approval of significant corporate transactions. This concentration of voting power makes it less likely that any
other holder of common stock will be able to affect the way we are managed or directors of our business and could delay or prevent an acquisition of us on terms that other stockholders may desire. In addition, if Warburg Pincus acquires additional
shares of our common stock such that it holds a majority of our common stock, Warburg Pincus would be able to control all matters submitted to our stockholders for approval, as well as our management and affairs. For example, in such instance,
Warburg Pincus would control the election of directors and approval of any merger, consolidation, sale of all or substantially all of our assets or other business combination or reorganization. In addition, if Warburg Pincus were to hold a majority
of our common stock, we would be deemed a controlled company for purposes of the NASDAQ listing requirements. Under the NASDAQ rules, a controlled company may elect not to comply with certain NASDAQ corporate governance
requirements, including (i) the requirement that a majority of our board of directors consist of independent directors, (ii) the requirement that the compensation of our officers be determined or recommended to the board by a majority of
independent directors or a compensation committee that is composed entirely of independent directors, and (iii) the requirement that director nominees be selected or recommended to the board by a majority of independent directors or a
nominating committee that is composed of entirely independent directors.

Furthermore, the interests of Warburg Pincus may not always
coincide with your interests or the interests of other stockholders and Warburg Pincus may act in a manner that advances their best interests and not necessarily those of other stockholders, including seeking a premium value for their common stock,
and might affect the prevailing market price for our common stock. Pursuant to our fourth amended and restated securityholders agreement, dated January 10, 2011, we are required to nominate and use our best efforts to elect to our board of directors
up to three individuals designated by an affiliate of Warburg Pincus, as more specifically described in Description of Capital Stock  Voting Rights. Our board of directors, which currently consists of six directors and one vacancy,
has the power to set the number of directors on our board from time to time.

We have broad discretion in the use of net proceeds from this
offering and may not use them effectively.

Although we currently intend to use the net proceeds from this offering in the manner
described in Use of Proceeds elsewhere in this prospectus, we will have broad discretion in the application of the net proceeds. Our failure to apply these funds effectively could affect our ability to continue to develop and eventually
to manufacture and sell our products.

Being a public company will increase our expenses and administrative burden.

As a public company, we will incur significant legal, accounting and other expenses that we did not incur as a private company. In addition, our
administrative staff will be required to

perform additional tasks. For example, in anticipation of becoming a public company, we will need to adopt additional internal controls and disclosure controls and procedures, retain a transfer
agent, adopt an insider trading policy and bear all of the internal and external costs of preparing and distributing periodic public reports in compliance with our obligations under the securities laws.

In addition, laws, regulations and standards applicable to public companies relating to corporate governance and public disclosure, including the
Sarbanes-Oxley Act and related regulations implemented by the Securities and Exchange Commission and the NASDAQ Global Market, are creating uncertainty for public companies, increasing legal and financial compliance costs and making some activities
more time consuming. However, for as long as we remain an emerging growth company as defined in the JOBS Act, we may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies
that are not emerging growth companies including, but not limited to, not being required to comply with the auditor attestation requirements of Section 404 of the Sarbanes-Oxley Act, reduced disclosure obligations regarding executive
compensation in our periodic reports and proxy statements, and exemptions from the requirements of holding a nonbinding advisory vote on executive compensation and shareholder approval of any golden parachute payments not previously approved. We may
take advantage of these reporting exemptions until we are no longer an emerging growth company. We will remain an emerging growth company for up to five years, although if the market value of our common stock that is held by
non-affiliates exceeds $700 million as of any June 30 before that time, we would cease to be an emerging growth company as of the following December 31.

We are currently evaluating and monitoring these rules and proposed changes to rules, and cannot predict or estimate the amount of additional costs
we may incur or the timing of such costs. These laws, regulations and standards are subject to varying interpretations, in many cases due to their lack of specificity, and, as a result, their application in practice may evolve over time as new
guidance is provided by regulatory and governing bodies. This could result in continuing uncertainty regarding compliance matters and higher costs necessitated by ongoing revisions to disclosure and governance practices. We intend to invest
resources to comply with evolving laws, regulations and standards, and this investment will result in increased general and administrative expenses and may divert managements time and attention from product development activities. If our
efforts to comply with new laws, regulations and standards differ from the activities intended by regulatory or governing bodies due to ambiguities related to practice, regulatory authorities may initiate legal proceedings against us and our
business may be harmed. In connection with this offering, we are increasing our directors and officers insurance coverage which will increase our insurance cost. In the future, it may be more expensive for us to obtain director and
officer liability insurance, and we may be required to accept reduced coverage or incur substantially higher costs to obtain coverage. These factors could also make it more difficult for us to attract and retain qualified members of our board of
directors, particularly to serve on our audit committee and compensation committee, and qualified executive officers.

Purchasers in this offering
will experience immediate and substantial dilution in the book value of their investment.

The initial public offering price will
be substantially higher than the net tangible book value per share of shares of our common stock based on the total value of our tangible assets less our total liabilities immediately following this offering. Therefore, if you purchase shares of our
common stock in this offering, you will experience immediate and substantial dilution of $9.47 per share in the price you pay for shares of our common stock as compared to its pro forma as

adjusted net tangible book value, assuming an initial public offering price of $13.00 per share, the mid-point of the price range set forth on the cover page of this prospectus. To the extent
outstanding options to purchase shares of common stock that are in the money are exercised, there will be further dilution. For further information on this calculation, see Dilution elsewhere in this prospectus.

A significant portion of our total outstanding shares of common stock is restricted from immediate resale but may be sold into the market in the near future.
This could cause the market price of our common stock to drop significantly, even if our business is doing well.

Sales of a
substantial number of shares of our common stock in the public market could occur in the future. These sales, or the perception in the market that the holders of a large number of shares of common stock intend to sell shares, could reduce the market
price of our common stock. After this offering, we will have 15,664,597 outstanding shares of common stock based on the number of shares outstanding as of March 31, 2012, assuming an initial public offering price of $13.00 per share, the mid-point
of the price range set forth on the cover page of this prospectus, and that the conversion of our preferred stock and accumulated dividends thereon and convertible notes and accrued interest thereon into common stock occurs on May 8, 2012. Of
these shares, 5,770,400 shares, excluding any shares purchased by our affiliates, may be resold in the public market immediately and the remaining 9,894,197 shares are currently restricted under securities laws or as a result of lock-up
agreements but will be able to be resold after this offering as described in the Shares Eligible for Future Sale section of this prospectus. Moreover, after this offering, holders of an aggregate of 9,878,775 shares of our common stock
will have rights, subject to certain conditions, to require us to file registration statements covering their shares or to include their shares in registration statements that we may file for ourselves or other stockholders. We also intend to
register all 2,663,857 shares of common stock that we may issue under our equity compensation plans. Once we register these shares, they can be freely sold in the public market upon issuance and once vested, subject to the 180 day lock-up
periods under the lock-up agreements described in the Underwriting section of this prospectus.

Future sales and issuances of our
common stock or rights to purchase common stock pursuant to our equity incentive plans could result in additional dilution of the percentage ownership of our stockholders and could cause our share price to fall.

We expect that significant additional capital will be needed in the future to continue our planned operations. To the extent we raise additional
capital by issuing equity securities, our stockholders may experience substantial dilution. We may sell common stock, convertible securities or other equity securities in one or more transactions at prices and in a manner we determine from time to
time. If we sell common stock, convertible securities or other equity securities in more than one transaction, investors may be materially diluted by subsequent sales. Such sales may also result in material dilution to our existing stockholders, and
new investors could gain rights superior to our existing stockholders.

As of March 31, 2012, we had options to purchase 3,857
shares outstanding under our 2001 Stock Option and Incentive Plan, or 2001 Stock Plan, and options to purchase 26 shares outstanding under our 2011 Equity Incentive Plan. We are authorized to grant equity awards, including stock options, to our
employees, directors and consultants, covering up to an aggregate of 2,500,000 shares of our common stock, pursuant to our 2011 Equity Incentive Plan which includes the increase in the number of shares reserved under our 2011 Equity Incentive Plan
in April 2012. Pursuant to the terms of our Bonus Plan and our Non-Employee Director

Bonus Plan, we expect to grant restricted stock units for 653,826 additional shares and 16,346 additional shares, respectively, of our common stock under our 2011 Equity Incentive Plan in
connection with this offering. We have also agreed to grant an option to purchase an expected 156,768 shares of common stock to our newly appointed Chief Development Officer under our 2011 Equity Incentive Plan in connection with this offering. In
addition, there will be 160,000 shares of our common stock available for issuance under our 2012 Employee Stock Purchase Plan. We plan to register the number of shares available for issuance under our 2001 Stock Plan, 2011 Equity Incentive Plan and
2012 Employee Stock Purchase Plan. Sales of such shares may result in material dilution to our existing stockholders, which could cause our share price to fall.

If securities or industry analysts do not publish research or publish inaccurate or unfavorable research about our business, our share price and trading volume could decline.

The trading market for our common stock will depend on the research and reports that securities or industry analysts publish about us or our
business. We do not have any control over these analysts. There can be no assurance that analysts will cover us or provide favorable coverage. If one or more of the analysts who cover us downgrade our stock or change their opinion of our stock, our
share price would likely decline. If one or more of these analysts cease coverage of our company or fail to regularly publish reports on us, we could lose visibility in the financial markets, which could cause our share price or trading volume to
decline.

Some provisions of our charter documents and Delaware law may have anti-takeover effects that could discourage an acquisition of us by
others, even if an acquisition would be beneficial to our stockholders, and may prevent attempts by our stockholders to replace or remove our current management.

Provisions in our amended and restated certificate of incorporation and bylaws, as well as provisions of Delaware law, could make it more difficult for a third party to acquire us or increase the cost of acquiring
us, even if doing so would benefit our stockholders. These provisions include:



authorizing the issuance of blank check convertible preferred stock, the terms of which may be established and shares of which may be issued without
stockholder approval;

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limiting the removal of directors by the stockholders;

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creating a staggered board of directors;

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prohibiting stockholder action by written consent, thereby requiring all stockholder actions to be taken at a meeting of our stockholders;

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eliminating the ability of stockholders to call a special meeting of stockholders;

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permitting our board of directors to accelerate the vesting of outstanding option grants upon certain transactions that result in a change of control; and

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establishing advance notice requirements for nominations for election to the board of directors or for proposing matters that can be acted upon at stockholder
meetings.

These provisions may also frustrate or prevent any attempts by our stockholders to replace or remove our
current management or members of our board of directors. In addition, we are subject to Section 203 of the Delaware General Corporation Law, which generally prohibits a Delaware corporation from engaging in any of a broad range of business
combinations with an interested stockholder for a period of three years following the date on which the stockholder

became an interested stockholder, unless such transactions are approved by our board of directors. This provision could have the effect of delaying or preventing a change of control, whether or
not it is desired by or beneficial to our stockholders. Further, other provisions of Delaware law may also discourage, delay or prevent someone from acquiring us or merging with us.

We do not anticipate paying cash dividends, and accordingly, stockholders must rely on stock appreciation for any return on their investment.

We do not anticipate paying cash dividends in the future. As a result, only appreciation of the market price of our common stock, which may never
occur, will provide a return to stockholders. Investors seeking cash dividends should not invest in our common stock.

Our ability to use our net
operating loss carryforwards and certain other tax attributes may be limited.

As of December 31, 2011, we had federal net operating
loss carryforwards of $203.8 million which will begin to expire in 2021 and federal research and development tax credit carryforwards of $7.4 million which will begin to expire in 2021. Our ability to utilize our federal net operating losses and
federal tax credits may be limited under Sections 382 and 383 of the Internal Revenue Code. The limitations apply if an ownership change, as defined by Section 382, occurs. Generally, an ownership change occurs when certain shareholders
increase their aggregated ownership by more than 50 percentage points over their lowest ownership percentage in a testing period (typically three years). We may already be subject to Section 382 limitations due to previous ownership changes. In
addition, future changes in stock ownership may also trigger an ownership change and, consequently, a Section 382 limitation. Due to the significant complexity and cost associated with a change in control study, and the expectation of
continuing to incur losses whereby the net operating losses and federal tax credits are not anticipated to be used in the foreseeable future, we have not assessed whether there have been changes in control since our formation. If we have experienced
changes in control at any time since our formation, utilization of its net operating losses or research and development credit carryforwards would be subject to annual limitations under Section 382. Any limitation may result in expiration of a
portion of the net operating loss or research and development credit carryforwards before utilization which would reduce our gross deferred tax assets and corresponding valuation allowance. As a result, if we earn net taxable income, our ability to
use our pre-change net operating loss carryforwards to offset United States federal taxable income may be subject to significant limitations, which could potentially result in increased future tax liability to us.

This prospectus contains forward-looking statements. All statements other than statements of historical facts contained in this prospectus,
including statements regarding our strategy, future operations, future financial position, future revenue, projected costs, prospects, plans, objectives of management and expected market growth are forward-looking statements. These statements
involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the
forward-looking statements.

the size and growth of the potential markets for our product candidates and our ability to serve those markets;



the rate and degree of market acceptance of any future products;



the success of competing drugs that are or become available; and



the loss of key scientific or management personnel.

These forward-looking statements are only predictions and we may not actually achieve the plans, intentions or expectations disclosed in our forward-looking statements, so you should not place undue reliance on our
forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements we make. We have based these forward-looking statements largely on our current
expectations and projections about future events and trends that we believe may affect our

business, financial condition and operating results. We have included important factors in the cautionary statements included in this prospectus, particularly in the Risk Factors
section, that could cause actual future results or events to differ materially from the forward-looking statements that we make. Our forward-looking statements do not reflect the potential impact of any future acquisitions, mergers, dispositions,
joint ventures or investments we may make.

Our forward-looking statements in this prospectus represent our views only as of the date of
this prospectus. We disclaim any intent or obligation to update forward-looking statements made in this prospectus to reflect changed assumptions, the occurrence of unanticipated events or changes to future operating results over time. You should,
therefore, not rely on these forward-looking statements as representing our views as of any date subsequent to the date of this prospectus.

We estimate that our net proceeds from the sale of 5,770,000 shares of common stock in this offering will be approximately $66.2 million after
deducting estimated offering expenses and underwriting discounts and commissions and assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover of this prospectus. If the over-allotment
option is exercised in full, we estimate that our net proceeds will be approximately $76.7 million. A $1.00 increase (decrease) in the assumed initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover
page of this prospectus, would increase (decrease) the net proceeds to us from this offering by $5.4 million, assuming the number of shares offered by us, as set forth on the cover page of this prospectus, remains the same and after deducting
estimated underwriting discounts and estimated offering expenses payable by us.

The principal purposes of this offering are to obtain
additional capital to support our operations, to create a public market for our common stock and to facilitate our future access to the public equity markets. We intend to use the net proceeds from this offering as follows:



approximately $32.0 million to fund the remaining estimated cost of our first planned Phase 3 clinical trial with the IV formulation of delafloxacin for the
treatment of ABSSSI;



approximately $16.0 million to fund ongoing research and development activities for our RX-04, RX-05 and RX-06 programs;



approximately $9.7 million to pay scheduled principal and interest through April 2014 under our loan agreement with Oxford Finance LLC bearing interest at a
rate of 9.1% per annum and maturing on June 1, 2015, the proceeds from which have been used and will continue to be used to fund our ongoing operations through the consummation of this offering and, following this offering, for general
corporate purposes; and



the remainder for working capital and other general corporate purposes, including for additional costs and expenses associated with being a public company.

We believe that the net proceeds from this offering, the amount we anticipate receiving under our collaboration
with Sanofi, and our existing cash and cash equivalents, together with interest thereon, will be sufficient to fund the continued development of delafloxacin and RX-04 through the following events:



receipt of top-line data from our initial Phase 3 clinical trial of the IV dosage form of delafloxacin for the treatment of ABSSSI; and



identification of a clinical candidate from the RX-04 program and submission of an Investigational New Drug, or IND, application.

The amount and timing of our actual expenditures will depend upon numerous factors, including the ongoing status and results of the initial Phase 3
clinical trial for delafloxacin and progress on the RX-04 program in collaboration with Sanofi. In particular, we will need to obtain additional funding beyond the proceeds of this contemplated offering in order to continue to advance the
development of radezolid.

Our expected use of net proceeds from this offering represents our current intentions based upon our present
plans and business condition. As of the date of this prospectus, we cannot predict with certainty all of the particular uses for the net proceeds to be received upon the completion of this offering or the amounts that we will actually spend on the
uses set forth above. The amounts and timing of our actual use of net proceeds will vary depending on numerous factors, including our ability to obtain additional financing, the relative success and

cost of our research, preclinical and clinical development programs, the amount and timing of revenues, if any, received from our collaboration with Sanofi and whether we are able to enter into
anticipated future collaborations. As a result, management will have broad discretion in the application of the net proceeds, and investors will be relying on our judgment regarding the application of the net proceeds of this offering. In addition,
we might decide to postpone or not pursue other clinical trials or any number of our research and development programs if the proceeds from this offering and the other sources of cash are less than expected.

Pending their use, we plan to invest the net proceeds from this offering in short- and intermediate-term, interest-bearing obligations,
investment-grade instruments, certificates of deposit or direct or guaranteed obligations of the United States government.

We have never paid or declared any cash dividends on our common stock, and we do not anticipate paying any cash dividends on our common stock in
the foreseeable future. In addition, certain of our outstanding warrants and our secured loans contain restrictions on the payment of dividends. We intend to retain all available funds and any future earnings to fund the development and expansion of
our business. Any future determination to pay dividends will be at the discretion of our board of directors and will depend upon a number of factors, including our results of operations, financial condition, future prospects, contractual
restrictions, restrictions imposed by applicable law and other factors our board of directors deems relevant.

The following table sets forth our cash and cash equivalents and capitalization as of December 31, 2011:



on an actual basis;



on an unaudited pro forma basis to give effect to the (i) issuance of 63,583 shares of common stock upon the conversion of all outstanding shares of our
convertible preferred stock and accumulated dividends thereon and 9,392,524 shares upon the conversion of all outstanding principal and accrued interest on the convertible notes payable assuming an initial public offering price per share of $13.00,
the mid-point of the range set forth on the cover page of this prospectus, (ii) settlement of the put rights upon conversion of the convertible notes payable, (iii) conversion of the preferred stock warrants into common stock warrants, and (iv)
elimination of the common stock warrant exercise price protection term, assuming in all cases that each had occurred on December 31, 2011; and



on an unaudited pro forma as adjusted basis to give effect to the (i) issuance of 63,583 shares of common stock upon the conversion of all outstanding
shares of our convertible preferred stock and accumulated dividends thereon and 9,392,524 shares upon the conversion of all outstanding principal and accrued interest on the convertible notes payable assuming an initial public offering price per
share of $13.00, the mid-point of the range set forth on the cover page of this prospectus, (ii) settlement of the put rights upon conversion of the convertible notes payable, (iii) conversion of the preferred stock warrants into common
stock warrants, (iv) elimination of the common stock warrant exercise price protection term, and (v) sale of 5,770,000 shares of common stock in this offering at an assumed initial public offering price of $13.00 per share, the mid-point
of the price range set forth on the cover page of this prospectus, after deducting estimated underwriters discounts and commissions and estimated offering expenses payable by us, assuming in all cases that each had occurred on December 31, 2011.

You should read this table together with our financial statements and the related notes thereto, as
well as the information under Selected Financial Data and Managements Discussion and Analysis of Financial Condition and Results of Operations. The unaudited pro forma and pro forma as adjusted information below is
prepared for illustrative purposes only and our capitalization following the completion of this offering will be adjusted based on the actual initial public offering price, the closing of the offering made hereby and other terms of the offering
determined at pricing.

The above table does not reflect the impact of $15,000 we borrowed under a loan and security agreement entered into in February 2012. As a result, our cash and cash equivalents
balance as of March 31, 2012 was $14,276. The aggregate principal amount outstanding under the loan and security agreement as of March 31, 2012 was $15,000. See Note 17 to our audited financial statements included elsewhere in this
prospectus for further details regarding this loan and security agreement.

(2)

The pro forma balance sheet data in the table above gives effect to the elimination of $175 of unamortized debt issuance costs upon conversion of the convertible notes payable to
stockholders as described in the paragraph above.

(3)

A $1.00 increase (decrease) in the assumed initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus,
would increase (decrease) each of the pro forma as adjusted cash and cash equivalents, additional paid-in capital, total stockholders equity and total capitalization by $5,366, assuming the shares offered by us as set forth on the cover of
this prospectus remain the same and after deducting the estimated underwriters discounts and commissions and estimated offering costs payable by us.

(4)

The pro forma as adjusted balance sheet data in the table above gives effect to the reclassification of $1,851 of deferred initial public offering costs recognized as of December
31, 2011 that will be charged to additional paid-in capital as a reduction of proceeds received in connection with this offering. As of December 31, 2011, we had made payments totaling $237 and had accrued $1,614 related to these costs.

The number of shares of our common stock to be outstanding after this offering is based
on 9,457,915 shares outstanding as of December 31, 2011. It does not include:



4,015 shares of our common stock issuable upon the exercise of stock options outstanding as of December 31, 2011 at a weighted average exercise price of $638.51
per share;



653,826 shares of our common stock issuable upon the vesting of restricted stock units granted under our 2011 Equity Incentive Plan pursuant to our Bonus Plan in
connection with this offering, assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus;



16,346 shares of our common stock issuable upon the vesting of restricted stock units granted under our 2011 Equity Incentive Plan pursuant to our Non-Employee
Director Bonus Plan in connection with this offering, assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus;



3,540 additional shares of our common stock that were available for future issuance as of December 31, 2011 under our 2011 Equity Incentive Plan; and



6,341 shares of our common stock issuable upon the exercise of warrants outstanding as of December 31, 2011 at a weighted average exercise price of $1,010.37 per
share.

If you invest in our common stock, your ownership interest will be diluted to the extent of the difference between the initial public offering
price and the pro forma as adjusted net tangible book value per share of our common stock immediately after the completion of this offering. Dilution results from the fact that the initial public offering price is substantially in excess of the net
tangible book value (deficit) per share attributable to the existing stockholders for the presently outstanding stock.

Our
historical net tangible book value (deficit) as of December 31, 2011 was $(241.3) million, or $(133,475) per share of common stock. Historical net tangible book value (deficit) per share represents the amount of our total tangible assets less
total liabilities and convertible preferred stock, divided by 1,808, the shares of common stock outstanding as of December 31, 2011.

Our pro forma net tangible book value (deficit) as of December 31, 2011 was $(14.3) million, or $(1.51) per share of common stock. Pro forma
net tangible book value (deficit) per share represents the amount of our total tangible assets less our total liabilities, divided by 9,457,915, the number of shares of our common stock outstanding, as of December 31, 2011, after giving effect to
the (i) issuance of 63,583 shares of our common stock upon the conversion of all outstanding shares of our convertible preferred stock and accumulated dividends thereon, (ii) issuance of 9,392,524 shares of our common stock upon conversion
of all outstanding principal and accrued interest on the convertible notes payable assuming an initial public offering price per share of $13.00, the mid-point of the range set forth on the cover page of this prospectus, (iii) settlement of the
put rights upon conversion of the convertible notes payable, (iv) conversion of the preferred stock warrants into common stock warrants and (v) elimination of the common stock warrant exercise price protection term, in all cases assuming
each occurred on December 31, 2011.

Investors participating in this offering will incur immediate and substantial dilution. After
giving effect to the sale of 5,770,000 shares of our common stock in this offering, assuming an initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus, and after deducting
estimated underwriters discounts and commissions and estimated offering expenses payable by us, our pro forma as adjusted net tangible book value as of December 31, 2011 would have been $53.8 million, or $3.53 per share. This amount
represents an immediate increase in pro forma as adjusted net tangible book value of $5.04 per share to our existing stockholders and an immediate dilution in pro forma as adjusted net tangible book value of $9.47 per share to investors
participating in this offering. We determine dilution by subtracting the pro forma as adjusted net tangible book value per share after this offering from the amount of cash that an investor participating in this offering paid for a share of common
stock.

Historical net tangible book value (deficit) per share as of December 31, 2011

$

(133,475

)

Increase in net tangible book value per share attributable to the issuance of common stock upon the conversion of all convertible preferred
stock and accrued dividends thereon and all outstanding principal and accrued interest on the convertible notes payable, the conversion of all preferred stock warrants to common stock warrants, the elimination of the common stock warrant exercise
price protection term, and the settlement of the put rights upon conversion of the convertible notes payable

133,474

Pro forma net tangible book value (deficit) per share as of December 31, 2011 before this offering

(1.51

)

Increase in pro forma net tangible book value per share attributable to cash payments by investors participating in this
offering

5.04

Pro forma as adjusted net tangible book value per share after this offering

3.53

Dilution in pro forma as adjusted net tangible book value per share to investors participating in this offering

$

9.47

If the underwriters exercise their option to purchase additional shares in full, the pro forma as adjusted
net tangible book value per share after giving effect to this offering would be $4.00 per share. This represents an increase in pro forma as adjusted net tangible book value of $5.51 per share to existing stockholders and dilution in pro
forma as adjusted net tangible book value of $9.00 per share to investors participating in this offering.

A $1.00 increase
(decrease) in the assumed initial public offering price per share of $13.00, the mid-point of the price range set forth on the cover page of this prospectus, would increase (decrease) our pro forma as adjusted net tangible book value after this
offering by $5.4 million and the pro forma as adjusted net tangible book value per share after this offering by $0.35 per share, and would increase (decrease) the dilution per share to investors participating in this offering by
$0.65 per share, in each case, assuming the number of shares offered by us, as set forth on the cover page of this prospectus, remains the same, after deducting the estimated underwriting discounts and commissions and estimated offering cost
payable by us. The information discussed above is illustrative only and will adjust based on the actual initial public offering price and other terms of this offering determined at pricing.

The following table summarizes, on a pro forma as adjusted basis as described above as of December
31, 2011, the differences between the number of shares purchased from us, the total consideration paid to us, and the average price per share paid to us by existing stockholders and by investors participating in this offering at an assumed initial
public offering price per share of $13.00, the mid-point of the range set forth on the cover page of this prospectus and before deducting estimated underwriting discounts and commissions and estimated offering costs payable by us.

Shares Purchased

Total Consideration

Average Priceper Share

Number

Percentage

Amount

Percentage

Existing stockholders

9,457,915

62.1

%

$

309,197,715

80.5

%

$

32.69

Investors participating in this offering

5,770,000

37.9

75,010,000

19.5

$

13.00

Total

15,227,915

100.0

%

$

384,207,715

100.0

%

$

25.23

A $1.00 increase (decrease) in the assumed initial public offering price per share of $13.00, the
mid-point of the price range set forth on the cover page of this prospectus, would increase (decrease) the total consideration paid by investors participating in this offering by $5.8 million, and increase (decrease) the percentage of total
consideration paid to us by investors participating in this offering by 1.19%, before deducting estimated underwriting discounts and estimated offering expenses payable by us, and assuming the number of shares offered by us, as set forth on the
cover page of this prospectus, remains the same.

The discussion and table above assume no exercise of the underwriters option to
purchase additional shares. If the underwriters option to purchase additional shares is exercised in full, the number of shares of our common stock held by existing stockholders will be further reduced to 59% of the total number of shares
of our common stock to be outstanding after this offering, and the number of shares of our common stock held by investors participating in this offering will be further increased to 41% of the total number of shares of our common stock to be
outstanding after this offering.

In addition, except as noted, the above discussion and table assume no exercise of stock options or
warrants to purchase common stock after December 31, 2011. As of December 31, 2011, we had outstanding options to purchase a total of 4,015 shares of our common stock at a weighted-average exercise price of $638.51 per share, 6,170 shares of common
stock issuable upon the exercise of outstanding warrants at a weighted-average exercise price of $941.12 per share and 171 shares of common stock issuable upon the exercise of outstanding convertible preferred stock warrants at an exercise price of
$3,509.57 per common share. If all such options and warrants had been exercised as of December 31, 2011, pro forma as adjusted net tangible book value per share would have been $4.12 per share and dilution to investors participating in this offering
would be $8.88 per share. To the extent we grant options to our employees in the future and those options are exercised or other issuances of common stock are made, there will be further dilution to investors participating in this offering.

Certain of our existing stockholders and their affiliated entities have indicated an interest in purchasing up to approximately $20.0
million in shares of our common stock in this offering at the initial public offering price. However, because indications of interest are not binding agreements or commitments to purchase, the underwriters could determine to sell more, less or no
shares to any of these existing stockholders and any of these existing stockholders could determine to purchase more, less or no shares in this offering. The foregoing discussion and tables do not reflect any potential purchases by these existing
stockholders and their affiliated entities.

The following table sets forth our selected financial data for the periods, and as of the dates, indicated. You should read the following selected
financial data in conjunction with our audited financial statements and the related notes thereto included elsewhere in this prospectus and the Managements Discussion and Analysis of Financial Condition and Results of Operations
section of this prospectus.

We derived the statement of operations data for the years ended December 31, 2009, 2010 and 2011, and
the balance sheet data as of December 31, 2010 and 2011, from our audited financial statements that are included elsewhere in this prospectus. We derived the statement of operations data for the fiscal years ended December 31, 2007 and
2008, and the balance sheet data as of December 31, 2007, 2008 and 2009, from our audited financial statements that are not included in this prospectus.

The pro forma net loss per share and weighted average shares have been calculated to give effect to the (i) issuance of shares of common stock upon conversion of all
outstanding shares of our convertible preferred stock and accumulated dividends thereon and upon conversion of all outstanding principal and accrued interest on the convertible notes payable assuming an initial public offering price per share of
$13.00, the mid-point of the range set forth on the cover page of this prospectus, (ii) settlement of the put rights upon the conversion of the convertible notes payable, (iii) conversion of the preferred stock warrants into common stock
warrants and (iv) elimination of the common stock warrant exercise price protection term, in all cases, assuming each had occurred on the later of January 1, 2011 or where applicable, the issuance date of the convertible notes payable. See
Note 2 to our financial statements included elsewhere in the prospectus.

The balance sheet data does not reflect the impact of $15,000 we borrowed under a loan and security agreement entered into in February 2012. As a result, our cash and cash
equivalents balance as of March 31, 2012 was $14,276. The aggregate principal amount outstanding under the loan and security agreement as of March 31, 2012 was $15,000. See Note 17 to our audited financial statements included
elsewhere in this prospectus for further details regarding this loan and security agreement.

(2)

Convertible notes payable and accrued interest on convertible notes payable were long-term obligations as of December 31, 2010 and were current liabilities as of
December 31, 2009 and December 31, 2011.

(3)

Deferred revenue is related to the collaboration and license agreement with Sanofi. See Note 3 to our audited financial statements included elsewhere in this prospectus.

MANAGEMENTS DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND
RESULTS OF OPERATIONS

The following discussion and analysis of our financial condition and results of operations should be read
in conjunction with Selected Financial Data and our financial statements and related notes appearing elsewhere in this prospectus. In addition to historical information, this discussion and analysis contains forward-looking statements
that involve risks, uncertainties and assumptions. Our actual results may differ materially from those discussed below. Factors that could cause or contribute to such differences include, but are not limited to, those identified below, and those
discussed in the section titled Risk Factors included elsewhere in this prospectus.

Overview

We are a biopharmaceutical company developing new antibiotics to provide superior coverage, safety and convenience for the treatment of serious and
life-threatening infections. Our proprietary drug discovery platform, which is based on Nobel Prize-winning science, provides an atomic-level, three-dimensional understanding of interactions between drug candidates and their bacterial targets
and enables us to systematically engineer antibiotics with enhanced characteristics.

Our most advanced product candidate, delafloxacin,
is intended for use as an effective and convenient first-line therapy primarily in hospitals prior to the availability of a specific diagnosis. Unlike currently available first-line treatments, delafloxacin has the potential to offer
broad-spectrum coverage as a monotherapy, including for methicillin-resistant Staphylococcus aureus, or MRSA, with both intravenous and oral formulations. Delafloxacin has completed four Phase 2 clinical trials, including a Phase 2b clinical
trial for the treatment of acute bacterial skin and skin structure infections, or ABSSSI. Based on the results from the Phase 2b clinical trial, we plan to commence the first of two planned Phase 3 trials for the treatment of ABSSSI in the second
half of 2012. The timing of our second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this contemplated offering. Based on our current expectations regarding the availability of such funding and
subject to the results of these two trials, we anticipate submitting a New Drug Application for delafloxacin for the treatment of ABSSSI as early as the fourth quarter of 2014 and for additional indications thereafter.

Our second product candidate, radezolid, is a next-generation, IV/oral oxazolidinone, designed to be a potent antibiotic with a safety profile
permitting long-term treatment of resistant infections, including those caused by MRSA. We have completed two Phase 2 clinical trials of radezolid. We are also pursuing RX-04, our preclinical program partnered with Sanofi, S.A., which has
produced new classes of antibiotics designed to combat the most difficult-to-treat, multi-drug resistant Gram-positive and Gram-negative bacteria. In addition, our pipeline includes RX-05, an antibacterial discovery program, and RX-06, an antifungal
discovery program, both of which target newly discovered binding sites within ribosomes.

We have funded our operations primarily through
private placements of convertible preferred stock and convertible debt, upfront and milestone payments under our collaboration with Sanofi, government tax credit programs and research grants. Since our inception in October 2000 through
December 31, 2011, we have received an aggregate of $214.9 million in such funding, which includes:



$122.4 million from the sales of convertible preferred stock;



$71.0 million from the issuance of convertible notes and common stock warrants;

$19.0 million in upfront and milestone payments under our collaboration with Sanofi; and



$2.5 million in government tax credit payments and research grants.

Prior to 2007, we received an aggregate of $122.4 million from sales of Series A, Series B and Series C convertible preferred stock. The
holders of our convertible preferred stock are entitled to a cumulative dividend at the rate of 8.0% per annum. All of our outstanding convertible preferred stock and accumulated dividends thereon will convert into 65,333 shares of common
stock, assuming that such conversion occurs on May 8, 2012.

During 2009, 2010 and 2011, we borrowed $35.0 million, $15.0 million and
$21.0 million, respectively, through multiple issuances of convertible notes payable to stockholders and warrants for the purchase of our common stock, which we refer to as the 2009 Financing, 2010 Financing and 2011 Financing, respectively.
The convertible notes issued in the 2009 Financing, 2010 Financing and 2011 Financing, which we refer to as the 2009 Notes, 2010 Notes and 2011 Notes, respectively, accrue interest at a rate of 10% per annum. The outstanding principal and
accrued but unpaid interest on the convertible notes will automatically convert into 9,827,456 shares of common stock, assuming an initial public offering, or IPO, price per share of $13.00, the mid-point of the price range set forth on the cover
page of this prospectus, and that such conversion occurs on May 8, 2012. This conversion will occur prior to our grant of restricted stock units pursuant to our Bonus Plan and our Non-Employee Director Bonus Plan for 653,826 additional shares and
16,346 additional shares, respectively, of our common stock in connection with this offering. Each of the 2009, 2010 and 2011 Notes also contains a provision, or put right, that entitles the holders to receive specified preferential redemption
payments upon a change of control or liquidation. In addition, as part of the 2009 Financing, 2010 Financing and 2011 Financing, we issued warrants to purchase 6,072 shares of our common stock at a weighted-average exercise price of $899.91, which
unless otherwise exercised prior to the expiration of their respective 10-year terms by the holders thereof will remain outstanding following this offering. The warrants are entitled to anti-dilution protection if we subsequently issue additional
shares of common stock for consideration per share less than the respective warrant exercise prices. The put rights of the convertible notes and the anti-dilution provisions of the warrants have been deemed to result in derivative instruments which
require liability classification and mark-to-market accounting at each balance sheet date. These anti-dilution and put rights will terminate upon the closing of this offering.

In February 2012, we entered into a Loan and Security Agreement, or loan agreement, pursuant to which we borrowed an aggregate principal amount of
$15.0 million. We are obligated to make monthly interest only payments in arrears, at a rate of 9.1% per annum, for a period of nine months commencing on April 1, 2012. Commencing on January 1, 2013, and continuing on the first day of each month
through and including June 1, 2015, we will make consecutive equal monthly payments of principal and interest. We paid a 0.5% facility fee at the inception of the loan, and upon repayment of the total amount borrowed, we will be required to pay an
amount equal to 4.5% of the total amount borrowed, both of which will be recognized as additional interest expense over the term of the loan. Amounts due under the loan agreement may become immediately due and payable upon the occurrence of a
material adverse change, as defined under the loan agreement. Under the terms of the loan agreement, we are subject to operational covenants, including limitations on our ability to incur liens or additional debt, pay dividends, redeem stock, make
specified investments and engage in merger, consolidation or asset sale transactions, among other restrictions. Additionally, in February 2012, in connection with the issuance of secured promissory notes pursuant to the loan agreement in February

2012, we also issued warrants to purchase 1,889 shares of our common stock. These warrants have a seven-year term and are immediately exercisable at an exercise price of $396.95 per share.

In June 2011, we entered into an exclusive, three year worldwide research collaboration and license agreement with Sanofi for
novel classes of antibiotics resulting from our RX-04 program. During the three year research term, the parties will each conduct research on a best efforts basis with each party being responsible for its own assigned research and development
costs. Under the collaboration, we received in July 2011 a non-refundable, upfront payment of $10.0 million, and a payment of $9.0 million for the achievement of research milestones. In addition, we received an additional payment of
$3.0 million from Sanofi in January 2012 for the achievement of a research milestone. For each RX-04 product developed by Sanofi, we are eligible for up to $9.0 million in potential research milestone payments, up to $27.0 million in
potential development milestone payments relating to initiation of Phase 1, 2 and 3 clinical trials, up to $50.0 million in potential regulatory milestone payments relating to approvals in various jurisdictions including the United States, the
European Union and Japan, and up to $100.0 million in potential commercial milestone payments. We may also receive tiered percentage royalties of up to 10% on sales from products commercialized under the agreement, if any. Sanofi has the right to
develop an unlimited number of products provided that Sanofi exercises, during the research term, a development and commercialization option, or option, to obtain each licensed compound. Upon each option exercise, we will grant to Sanofi an
exclusive, worldwide, royalty-bearing license, with the right to grant sublicenses, to develop, market and sell the licensed compound, and Sanofi will assume responsibility for the development and commercialization of the licensed compound. We
retain all rights pertaining to the discovery platform and all future programs, and also have the right to opt into a co-development and co-commercialization arrangement with Sanofi, which provides an equal sharing of profits in the United States
for one product of our choice.

We have never been profitable and have incurred significant net losses since our inception. We incurred
net losses of $28.2 million, $26.8 million and $53.5 million for the years ended December 31, 2009, 2010 and 2011, respectively. These losses have resulted principally from costs incurred in connection with research and development activities,
general and administrative costs associated with our operations and interest expense in connection with our convertible notes payable. As of December 31, 2011, we had an accumulated deficit of $244.3 million and cash and cash equivalents of
$8.0 million.

We expect to continue to incur operating losses for the next several years as we work to discover, develop and
commercialize our product candidates. As a result, we will seek to fund our operations through public or private equity offerings, debt financings and corporate collaborations and licensing arrangements. We cannot ensure that such funds will be
available on terms favorable to us, if at all. The terms of any financing may adversely affect the holdings or rights of our stockholders and debt holders. Arrangements with collaborators or others may require us to relinquish rights to certain of
our technologies or product candidates. In addition, we may never successfully complete development of any of our product candidates, obtain adequate patent protection for our technology, obtain necessary regulatory approval for our product
candidates or achieve commercial viability for any approved product candidates. If we are not able to raise additional capital on terms acceptable to us, or at all, as and when needed, we may be required to curtail our operations, and we may be
unable to continue as a going concern. Our cash and cash equivalent balances as of December 31, 2011, significant debt outstanding, net capital deficiency and recurring losses from operations raise substantial doubt about our ability to
continue as a going concern. As a result, our independent registered public accounting firm included an explanatory paragraph in its report on our financial statements as of and for the year ended December 31, 2011 with respect to this
uncertainty.

Our 2011 revenues are solely comprised of contract revenues resulting from our
collaboration with Sanofi. In July 2011, in connection with our collaboration, we received a non-refundable, upfront payment of $10.0 million, and a payment of $9.0 million for the achievement of certain research milestones, the majority of which we
are recognizing over the three year research term. For the year ended December 31, 2011, we recognized $2.7 million of contract revenues under the collaboration. We estimate that as we continue to recognize revenues under the
collaboration, our contract revenues for the next twelve months will be approximately $9.0 million, including the recognition in January 2012 of contract revenues of $3.0 million as a result of the payment received for the achievement of a research
milestone. In the future, contract revenues under our collaboration with Sanofi may include additional payments for achieving research milestones, license payments for product candidates, as well as payments for such licensed product candidates
achieving development, regulatory and commercial milestones, and product royalties.

We have no products approved for sale, have not
generated any revenues from product sales since our inception and do not expect to generate any revenue from the sale of products in the near future. If our discovery or development efforts result in clinical success and regulatory approval or
collaboration agreements with third parties for any of our product candidates, we may generate revenues from those product candidates.

Research
and Development Expenses

The majority of our operating expenses to date have been for research and development activities
related to delafloxacin, radezolid and our discovery/preclinical programs. We record all research and development expenses, including those paid to third parties, to operations as incurred.

Research and development expenses consist primarily of costs associated with our product discovery and development efforts, including preclinical
and clinical trials. Research and development expenses include:



outsourced discovery and development expenses incurred through agreements with contract research organizations, or CROs, contract manufacturers and medicinal
chemistry service providers, and milestone and license payments made under licensing arrangements;

We
have been developing delafloxacin, radezolid and our discovery programs in parallel, and typically use our employee and infrastructure resources across multiple research and development programs. We track outsourced discovery and development costs
by specific discovery programs and development compounds but do not allocate personnel or other internal costs related to research and development to specific discovery programs or development compounds. These expenses are included in personnel
costs and other internal costs, respectively, in the table below.

The following table summarizes our research and development expenses for the years ended
December 31, 2009, 2010 and 2011:

Years Ended December 31,

2009

2010

2011

(In thousands)

Outsourced discovery and development costs:

Delafloxacin

$

3,896

$

2,935

$

19,123

Radezolid

3,605

141

2,485

RX-04

879

1,035

593

Other

2



402

Total outsourced discovery and development costs

8,382

4,111

22,603

Personnel costs

6,227

5,175

5,035

Other internal costs

2,983

3,136

3,568

Total research and development expenses

$

17,592

$

12,422

$

31,206

Since acquiring delafloxacin from Wakunaga Pharmaceutical Co., Ltd., or Wakunaga, in 2006 and through
December 31, 2011, we have incurred outsourced discovery and development costs for delafloxacin of approximately $48.0 million, including the initial license fee of $1.5 million paid in May 2006. Through December 31, 2011, we have incurred
outsourced discovery and development costs of approximately $30.8 million for radezolid and $3.3 million for RX-04. Through December 31, 2011, the outsourced discovery and development costs for other product candidates and preclinical and
discovery programs were immaterial.

The successful development of our clinical and preclinical product candidates is highly uncertain.
At this time, due to the inherently unpredictable nature of preclinical and clinical development, and given the early stage of our discovery programs, we cannot reasonably estimate or know the nature, specific timing or estimated costs of the
efforts that will be necessary to complete the development of our product candidates. However, we expect that our research and development expenses will increase significantly in future periods as we continue the clinical development of delafloxacin
and radezolid, and conduct research and development activities on our RX-04 preclinical program and our discovery programs. We expect to fund our research and development expenses from our cash and cash equivalents, a portion of the net proceeds
from this offering, milestone payments received from the collaboration with Sanofi, if any, additional financing transactions and collaboration arrangements that we intend to enter into. We cannot forecast with any degree of certainty which product
candidates or preclinical programs may be subject to future collaborations or contracts, when such arrangements will be secured, if at all, and to what degree such arrangements would affect our development plans and capital requirements.

Delafloxacin. We plan to commence the first of two planned Phase 3 trials for the treatment of ABSSSI in the second half of 2012. The timing
of our second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this contemplated offering. Based on our current expectations regarding the availability of such funding and subject to the results of
these two trials, we anticipate submitting applications for marketing approval to the U.S. Food and Drug Administration and the European Medicines Agency as early as the fourth quarter of 2014. We also intend to seek approval for additional
indications for delafloxacin, including CABP and cIAI.

Radezolid. Subject to obtaining sufficient additional funding beyond the
proceeds of this contemplated offering, we intend to initiate a Phase 2 study for the treatment of ABSSSI and a

Phase 1 long-term safety study in humans to demonstrate what we believe is a long-term safety advantage over Zyvox. Following these studies, we also intend to perform additional clinical
trials of radezolid in ABSSSI and CABP and for indications that require long-term treatment, such as osteomyelitis and prosthetic and joint infections.

RX-04. We intend to work with Sanofi under our collaboration agreement to identify and develop multiple RX-04 product candidates. In addition to the development and commercial milestone payments for which we
are eligible for each RX-04 product candidate, we intend to exercise our right to co-commercialize one RX-04 product of our choosing in the United States.

General and Administrative Expenses

General and administrative expenses consist
primarily of personnel costs, including salary, benefits and stock-based compensation, for employees in administration, finance and business development, as well as costs associated with recruitment efforts. Other significant expenses include: rent
and other facilities costs; professional and consulting fees for accounting and tax services, business development activities and general legal services, including legal expenses to pursue patent protection of our intellectual property; and travel
and other costs. We expect general and administrative expenses to increase significantly as we begin operating as a public company and continue to build our corporate infrastructure in support of continued development of delafloxacin, radezolid, our
RX-04 preclinical program and our discovery programs. These increases may impact: personnel costs; legal, accounting and consultant fees; expenses related to compliance with the Sarbanes-Oxley Act of 2002; expenses related to filing annual,
quarterly and other reports and documents with the Securities and Exchange Commission; increased directors and officers insurance premiums; fees for investor relations services; expenses related to listing and transfer agent fees; and
expenses for implementing enhanced business systems.

Interest Income

Interest income consists of interest earned on our cash and cash equivalents, though, since 2009, our interest income has not been significant due
to nominal cash and cash equivalent balances. We anticipate that our interest income will increase following the receipt of the proceeds from this offering.

Interest Expense

Interest expense consists of: cash interest paid or accrued on notes
payable and convertible notes payable; non-cash interest expense related to the amortization of debt issuance costs and debt discounts associated with the issuance of our notes payable and convertible notes payable; mark-to-market adjustments for
changes in value of the preferred stock warrants issued in connection with our notes payable; mark-to-market adjustments for changes in the value of the common stock warrants issued in connection with the issuance of convertible notes payable; and
mark-to-market adjustments for changes in the value of change of control and liquidation put rights associated with the issuance of our convertible notes payable. We anticipate that our interest expense will decrease significantly upon the
completion of this offering when the convertible notes payable convert into shares of common stock.

Other Income

Other income for the year ended December 31, 2010 included income related to the Qualifying Therapeutic Discovery Project, or QTDP, program,
which provided for reimbursement in 2010 of certain costs paid or incurred during 2009 and 2010 directly related to the conduct of a QTDP program. Other income for all periods also includes income received as a result of

legislation in the State of Connecticut where companies have the opportunity to exchange certain research and development tax credit carryforwards for a cash payment of 65% of the research and
development tax credit. We do not anticipate any further income related to the QTDP program, however, we will continue to record other income relating to the exchange of research and development tax credit carryforwards with the State of
Connecticut, to the extent that the program continues.

Income Taxes

As of December 31, 2011, we had federal and state net operating loss carryforwards of $203.8 million and $203.5 million,
respectively, and federal and state research and development tax credit carryforwards of $7.4 million and $3.5 million, respectively. Our federal and state net operating loss carryforwards and federal research and development tax credits
will expire through 2031 if not used, and our state research and development tax credit carryforwards do not expire.

The Tax
Reform Act of 1986 provides for a limitation on the annual use of federal net operating loss and research and development tax credit carryforwards following certain ownership changes, which could limit our ability to utilize these carryforwards. We
may already be subject to Section 382 limitations due to previous ownership changes. In addition, future changes in stock ownership may also trigger an ownership change and, consequently, a Section 382 limitation. Due to the significant
complexity and cost associated with a change in control study, and the expectation of continuing to incur losses whereby the net operating losses and federal tax credits are not anticipated to be used in the foreseeable future, we have not assessed
whether there have been changes in control since our formation. If we have experienced changes in control at any time since our formation, utilization of our net operating losses or research and development credit carryforwards would be subject to
significant annual limitations under Section 382.

Critical Accounting Policies and Significant Judgments and Estimates

Our managements discussion and analysis of financial condition and results of operations is based on our financial statements, which have been
prepared in accordance with accounting principles generally accepted in the United States. The preparation of these financial statements requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities, and
revenues and expenses during the reporting periods. On an ongoing basis, we evaluate our estimates and judgments, including those described below. We base our estimates on historical experience and on various other factors that we believe are
reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. Actual results may differ from these estimates under
different assumptions or conditions.

While our significant accounting policies are more fully described in Note 2 to our financial
statements appearing elsewhere in this prospectus, we believe that the following accounting policies are critical to the process of making significant judgments and estimates in the preparation of our financial statements.

Revenue Recognition

During 2011, we
entered into our first collaboration and license agreement with Sanofi for the research and development of novel classes of antibiotics under our RX-04 program The terms of the agreement include non-refundable upfront fees, and the potential for
research, development, regulatory and commercial milestone fees, as well as royalties on product sales of licensed products, if and when such product sales occur.

We recognize revenue when all of the following criteria are met: persuasive evidence of an arrangement
exists, services are performed or products have been delivered, the fee is fixed and determinable and collection is reasonably assured. Determinations of whether persuasive evidence of an arrangement exists and whether delivery has occurred or
services have been rendered are based on managements judgments regarding the fixed nature of the fees charged for deliverables and the collectability of those fees. Should changes in conditions cause management to determine that these criteria
are not met for any new or modified transactions, revenue recognized could be adversely affected.

provided guidance on how deliverables in an arrangement should be separated and how the arrangement consideration should be allocated to the separate units of
accounting;



required an entity to determine the selling price of a separate deliverable using a hierarchy of (i) vendor-specific objective evidence, or VSOE,
(ii) third-party evidence, or TPE, or (iii) best estimate of selling price, or BESP; and



required the allocation of the arrangement consideration, at the inception of the arrangement, to the separate units of accounting based on relative fair value.

We evaluate all deliverables within an arrangement to determine whether or not they provide value on a stand-alone
basis. Based on this evaluation, the deliverables are separated into units of accounting. The arrangement consideration that is fixed and determinable at the inception of the arrangement is allocated to the separate units of accounting based on
relative fair value. We may exercise significant judgment in determining whether a deliverable is a separate unit of accounting, as well as in estimating the selling prices of such unit of accounting.

To determine the selling price of a separate deliverable, we use the hierarchy as prescribed in ASC Topic 605-25 based on VSOE, TPE or BESP. VSOE
is based on the price charged when the element is sold separately and is the price actually charged for that deliverable. TPE is determined based on third party evidence for a similar deliverable when sold separately and BESP is the price at which
we would transact a sale if the elements of collaboration and license arrangements were sold on a stand-alone basis. We expect that establishing VSOE or TPE for the deliverables within collaboration and license arrangements will be difficult as we
do not have a history of entering into such arrangements or selling the individual deliverables within such arrangements separately. In addition, there is significant differentiation in these arrangements, which indicates that comparable third party
pricing may not be available. We expect the selling price for the deliverables within collaboration and license arrangements to be determined using BESP. The process for determining BESP involves significant judgment on our part and includes
consideration of multiple factors such as estimated direct expenses and other costs, and available data.

For each unit of accounting
identified within an arrangement, we determine the period over which the performance obligation occurs. Revenue is then recognized using either a proportional performance or straight-line method. We recognize revenue using the proportional
performance method when the level of effort to complete our performance obligations under an

arrangement can be reasonably estimated and such performance obligations are provided on a best-efforts basis. Direct labor hours or full time equivalents are typically used as the measurement of
performance.

In connection with the collaboration with Sanofi, we were required to make numerous estimates and judgments, primarily
related to the determination of deliverables, unit(s) of accounting and BESP. In particular, based on our judgment, we identified that the deliverables under the collaboration were (i) the research license, (ii) research services during
the three year research term and (iii) joint steering committee, or JSC, participation, and determined that these deliverables should be accounted for as a single unit of accounting. We also concluded that the future ability by Sanofi to
exercise an option is a substantive option as it is in the control of Sanofi, and therefore it was not considered to be a deliverable at the inception of the collaboration. Finally, we determined that the BESP for the single unit of accounting
discussed above was $18.3 million by considering the number of personnel who will be dedicated to the research services and JSC participation during the three year research term, and the estimated costs of the personnel based on our annual
historical direct costs, together with a market-based profit margin, which was determined based on an analysis of third-party data for companies providing a similar type of outsourced scientific personnel services.

Additionally, we considered that after the completion of the three year research term, the collaboration contains a two year follow-on period in
which neither party will conduct any research or development activities on any RX-04 compound other than licensed compounds. Therefore, we determined that the remaining initial consideration of $0.7 million represents an upfront fee that will be
recognized as contract revenues on a straight-line basis over the customer benefit period, which is five years.

Effective
January 1, 2011, we adopted ASU No. 2010-17, Milestone Method of Revenue Recognition, or ASU 2010-17, which provides guidance on revenue recognition using the milestone method. Under the milestone method, a payment that is
contingent upon the achievement of a substantive milestone is recognized in its entirety in the period in which the milestone is achieved. The determination that a milestone is substantive is subject to considerable judgment. In January 2012, we
received a $3.0 million payment from Sanofi for the achievement of a research milestone. We determined that the milestone was substantive and therefore recognized the amount as contract revenues in its entirety in January 2012. If we receive
additional milestone payments in the future under the collaboration with Sanofi, we will recognize such payments under the milestone method.

Royalty revenues will be recognized based on contract terms when reported sales are reliably measurable and collectability is reasonably assured. To date, none of our products has been approved, and therefore we
have not earned any royalty revenue from product sales.

Research and Development

As part of the process of preparing our financial statements, we are required to estimate accrued and prepaid research and development expenses. We
review new and open contracts, and communicate with applicable internal and vendor personnel to identify services that have been performed on our behalf and estimate the level of service performed and the associated costs incurred for the service
when we have not yet been invoiced or otherwise notified of the actual cost for accrued expenses. The majority of our service providers invoice us monthly in arrears for services performed, however, some require advanced payments. We also review,
with applicable internal and vendor personnel, services that have been performed when payment was required in advance and estimate the level of service performed and the associated costs incurred. We make estimates of our accrued and prepaid
expenses as of each balance sheet date

in our financial statements based on facts and circumstances known to us. We also periodically confirm the accuracy of our estimates with the service providers and make adjustments, if necessary.
To date, we have not adjusted our estimates at any particular balance sheet date in any material amount. Examples of estimated accrued and prepaid expenses include:



fees paid to CROs in connection with preclinical studies and clinical trials;



fees paid to investigative sites in connection with clinical trials;



fees paid to contract manufacturers in connection with the production of clinical trial materials;



fees paid for outsourced chemistry services;



obligations under licensing arrangements; and



professional service fees.

We
base our accrued and prepaid expenses related to preclinical studies and clinical trials on our estimates of the services received and efforts expended, all pursuant to contracts with multiple research institutions and CROs that conduct and manage
preclinical studies and clinical trials on our behalf. The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. Payments under some of these contracts depend on
factors such as the successful enrollment of patients and the completion of clinical trial milestones. We estimate the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of
the performance of services or the level of effort varies from our estimate, we adjust accordingly. If we do not identify costs that have been incurred or if we underestimate or overestimate the level of services performed or the costs of these
services, our actual expenses could differ from our estimates.

Stock-Based Compensation

We account for stock-based compensation by measuring and recognizing compensation expense for all stock-based awards made to employees and directors
based on grant date fair values. We use the straight-line method to allocate compensation cost to reporting periods over each optionees requisite service period, which is generally the vesting period. We estimate forfeitures at the time of
grant based on our historical experience and revise, if necessary, in subsequent periods if actual forfeitures differ from estimates. We use the Black-Scholes option-pricing model as the most appropriate fair-value method for our stock-based awards.
The Black-Scholes option-pricing model requires the input of subjective assumptions, including the expected volatility, the expected term and the fair value of the underlying common stock on the date of grant.

We account for all stock-based awards issued to non-employees based on their fair value on the measurement dates using the Black-Scholes
option-pricing model. Stock-based awards granted to non-employees are subject to periodic revaluation over their vesting terms. As a result, the charge to operations for non-employee options with vesting is affected each reporting period by changes
in the fair value of our common stock.

The following table summarizes our assumptions used in the Black-Scholes option-pricing model for the
years ended December 31, 2009, 2010 and 2011:

Years Ended December 31,

2009

2010

2011

Risk free interest rate

2.29% - 2.92%

2.51% - 2.83%

1.13% - 2.35%

Expected dividend yield

0%

0%

0%

Expected termemployee awards

6 years

6 years

6 years

Expected termnon-employee awards

10 years

N/A

10 years

Expected volatility

80%

76%

70%

Risk-free Interest Rate. The risk-free interest rate was based on zero coupon United States Treasury
instruments that had terms consistent with the expected term of our stock option grants.

Expected Dividend Yield. We have never
declared or paid any cash dividends and do not presently plan to pay cash dividends in the foreseeable future.

Expected Term. We
utilize the simplified method for plain vanilla options to estimate the expected term of stock option grants to employees. Under this approach, the expected term is presumed to be the simple average of the vesting term and
the contractual term of the option. We utilize the contractual term as the expected term for stock option grants to non-employees.

Expected Volatility. The expected volatility used to value stock option grants is based on volatilities of a peer group of similar companies
whose share prices are publicly available. The peer group was developed based on companies in the pharmaceutical and biotechnology industry in a similar stage of development.

Stock-Based Compensation Summary. Stock-based compensation for stock option grants is reported in our statements of operations for the years ended December 31, 2009, 2010 and 2011 as follows:

Years EndedDecember 31,

2009

2010

2011

(In thousands)

Research and development

$

406

$

160

$

59

General and administrative

544

162

198

Total stock-based compensation

$

950

$

322

$

257

Based on stock options outstanding as of December 31, 2011, we had unrecognized stock-based compensation
expense for employees, net of estimated forfeitures, of $0.3 million which will be recognized over a weighted-average period of 2.08 years.

Assuming an IPO price per share of $13.00, the mid-point of the range set forth on the cover of this prospectus, the intrinsic value of the 4,015 outstanding vested and unvested options at December 31, 2011
would have been $398 (in whole dollars).

We expect to continue to grant stock options in the future, which may increase our
stock-based compensation expense in future periods. The assumptions used above in the Black-Scholes option-pricing model represent managements best estimates, but these estimates involve inherent uncertainties and the application of
managements judgment. As a result, if factors change, and we use different assumptions, our stock-based compensation could be materially different in the future.

The following table shows the grant date, number of shares, exercise price per share and fair value
estimate per common share of stock options granted since January 1, 2011:

Grant Date

NumberofShares

Exercise Priceper Share

Fair ValueEstimate perCommon Share

January 31, 2011

35

$

396.95

$

7.94

March 19, 2011

4

$

34.02

$

7.94

September 1, 2011

75

$

9.07

$

9.07

November 18, 2011

26

$

9.07

$

9.07

Common Stock Fair Value

The fair value of our common stock underlying stock options granted has historically been determined by our board of directors, with assistance from management, based upon information available at the time of
grant. The intention has been that all options granted be exercisable at a price per share not less than the per share fair value of our common stock underlying those options on the date of grant. Given the absence of a public trading market for our
common stock, and in accordance with the American Institute of Certified Public Accountants Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation, management and our board of directors have exercised
reasonable judgment and considered numerous objective and subjective factors to determine the best estimate of the fair value of our common stock at each stock option grant date. These factors included:



the progress of our research and development programs, including the status of clinical trials for our products;



achievement of enterprise milestones, including our entering into collaboration and licensing agreements;



our financial condition, including cash on hand and debt levels;



our need for future financing to fund operations;



the composition of, and changes to, our management team and board of directors;



the rights and preferences of our convertible preferred stock and convertible notes payable relative to our common stock;



the lack of marketability of our common stock;



an analysis of mergers and acquisitions, IPOs, and the market performance of similar companies in the pharmaceutical and biotechnology industry sectors;



the likelihood of achieving a discrete liquidity event, such as a sale or merger, or IPO, given prevailing market conditions;

Grants 2011.Valuations of our common stock were completed as of December 31, 2010, June 30, 2011,
September 30, 2011 and December 31, 2011. During 2009, 2010 and 2011, we issued an aggregate of $71.0 million of convertible notes payable, with associated common stock warrants, with exercise prices not less than the per share fair value
of our common stock. The terms of the notes, as discussed further below and under Liquidity and Capital Resources, provided for varying economic outcomes for the debt holders depending on

differing types of liquidity events, such as an IPO or a strategic transaction. In addition, management and the board of directors determined that, given the stage of development for its programs
and the current outlook for them, there was a likelihood that there would be differing times to liquidity depending on whether the liquidity event would be an IPO or a strategic transaction.

We value our common stock using the probability-weighted expected return method, or PWERM. Under the PWERM, the value of a companys common
stock is estimated based upon an analysis of future enterprise values under various liquidity events. The future enterprise values are allocated among the various convertible debt and equity classes expected to be outstanding at the various
liquidity events based on the rights and preferences of each class. The future value of the common stock under each liquidation event is discounted back to the valuation date at an appropriate risk-adjusted discount rate and probability weighted to
arrive at an indication of value for the common stock. A discount for lack of marketability, to account for the illiquidity of the common stock, is applied to the indicated common stock value to determine the fair value of the common stock.

In connection with the PWERM analyses as of December 31, 2010, June 30, 2011, September 30, 2011 and December 31,
2011, two types of future event scenarios were considered: an IPO and a strategic sale or merger. Three different IPO and four different sale/merger scenarios were considered for a total of seven scenarios as of each valuation date, in order to
reflect a range of possible values. As of each valuation date, excluding December 31, 2011, our management and board of directors determined the total probability for the three IPO scenarios was 65%, with a corresponding total probability for
the four sale/merger scenarios of 35%, based on an analysis of current market conditions and management and the board of directors expectations of the timing of future scientific progress with its product candidates and discovery programs. For
the December 31, 2011 valuation date, our management and board of directors determined the total probability for the three IPO scenarios was 70%, with a corresponding total probability for the four sale/merger scenarios of 30%, based on an
analysis of current market conditions and management and the board of directors evaluation of the continued progress of the IPO process. The future enterprise value for each scenario was estimated by management and the board of directors based
on an analysis of IPOs or sales/mergers of companies in a similar stage of development as our own at each respective valuation date. For each scenario, the proceeds to the common stockholders were calculated based on the preferences and priorities
between the convertible debt and preferred and common stock.

For purposes of the December 31, 2010 valuation, a discount for lack
of marketability of 10% was applied to account for the lack of access to an active public market for the common stock and the fact that our common stock represents a minority interest in our company. Despite our early stage of development, we
determined a discount of 10% was appropriate after giving consideration to the recent completion of several studies for Phase 2 clinical trials which were moving us closer to a liquidity event. For purposes of the June 30, 2011,
September 30, 2011 and December 31, 2011 valuations, discounts of 5%, 5% and 2.5%, respectively, were applied for lack of marketability due to the passage of time resulting in a further reduction in the estimated time to an expected
liquidity event from prior valuations.

Management and the board of directors increased its estimate of our probability-weighted future
enterprise value as of December 31, 2010, as compared to December 31, 2009, based on the establishment of a final protocol for the delafloxacin Phase 2b study and the ongoing progress for a potential collaborative deal with RX-04. In connection with
the June 30, 2011 valuation, management and the board of directors further increased its estimate of our probability-weighted future enterprise value based on the significant changes in the business

from December 31, 2010 to June 30, 2011 as discussed below. For purposes of the September 30, 2011 valuation, as the Sanofi collaboration was still in its early stages and no
additional data was available from either the delafloxacin Phase 2b clinical trial or the radezolid long-term preclinical study, management and the board of directors used the same probability-weighted future enterprise value as was used for the
June 30, 2011 valuation. For purposes of the December 31, 2011 valuation, management and the board of directors decreased its estimate of our probability-weighted future enterprise value based on an analysis of then- current IPO valuations
of similarly-situated companies.

Based on these valuations, the fair value of our common stock as of December 31, 2010 was
determined to be $7.94, the fair value of our common stock as of June 30, 2011 and September 30, 2011 was determined to be $9.07, and the fair value of our common stock as of December 31, 2011 was determined to be $7.94.

The decrease in the fair value of our common stock from December 31, 2009 to December 31, 2010 can be mainly attributed to the
dilutive effect of the issuances during 2010 of $15.0 million of convertible notes payable, and the anticipated issuance in January 2011 of approximately $20.0 million of additional convertible notes payable, with associated common stock warrants.
The increase in the fair value of our common stock from December 31, 2010 to June 30, 2011 and September 30, 2011 can be attributed to the increase in the estimate of our probability-weighted future enterprise value which was based on the
following significant changes in the business during the period from December 31, 2010 to June 30, 2011: we signed an exclusive, worldwide research agreement with Sanofi for novel classes of antibiotics resulting from the RX-04 program for
the treatment of Gram-negative and Gram-positive bacteria; enrollment in the delafloxacin Phase 2b clinical trial was proceeding according to plan; and the radezolid long-term preclinical study was showing favorable evidence of radezolids
long-term safety profile. The decrease in the fair value of our common stock from September 30, 2011 to December 31, 2011 can be attributed to the decrease in the estimate of our probability-weighted future enterprise value based on an
analysis of then-current IPO valuations of similarly-situated companies.

Although the mid-point of the IPO price range set forth on
the cover page of this prospectus of $13.00 per share is greater than the $7.94 fair value of our common stock that was determined by our board of directors on December 31, 2011, there are several factors that explain this difference. We filed the
registration statement of which this prospectus is a part in November 2011 and our first amendment to the registration statement in December 2011. Subsequent to December 2011, we have filed three amendments to the registration statement, our second
amendment in January 2012, our third amendment in March 2012 and our fourth amendment in April 2012. In addition, in February 2012, we entered into a financing agreement with Oxford Finance LLC under which we borrowed $15 million to provide
liquidity through our anticipated IPO date. These facts signaled that an IPO scenario was becoming more likely, resulting in a significantly higher total probability for the IPO scenarios as compared to the sale/merger scenarios. Any increase in
probabilities assigned to the IPO scenarios rather than the sale/merger scenarios results in an increase in the fair value of our common stock. Furthermore, the determination of the estimated IPO price range necessarily assumes that the IPO has
occurred, a public market for our common stock has been created and that our preferred shares and convertible notes payable to stockholders have converted into common stock in connection with the IPO. The mid-point of the estimated range of the IPO
price also excludes any discount for lack of marketability for our common stock, which was appropriately taken into account in the boards fair market value determination as of December 31, 2011.

There are significant judgments and estimates involved in the determination of the above fair values of our common stock. These judgments and
estimates include assumptions, among

others, of: our future performance; the time to a liquidity event such as an IPO or a sale or merger; the probability and timing of our progress towards commercialization of our programs; the
valuation of the future cash flows from our programs; and the appropriate valuation methods used in determining fair value. In addition to the significant judgments, estimates and assumptions noted above, a significant factor that impacts the fair
valuation of our common stock is the conversion rights of our 2009 Notes, 2010 Notes and 2011 Notes. Such conversion rights provide that in the event of an IPO prior to maturity, the outstanding amounts of the 2009 Notes, 2010 Notes and 2011 Notes,
including accrued interest, shall automatically convert immediately prior to the IPO into such number of shares of common stock that represent up to 99.2% of our outstanding common stock on a fully-diluted basis, but with the ultimate percentage to
be determined by the offering price in the IPO and subject to the limitations of value equal to three times the outstanding principal and interest on the 2011 Notes and 2010 Notes and one and one-third times the outstanding principal and interest on
the 2009 Notes, as more fully described in Note 6 to our financial statements appearing elsewhere in this prospectus. Accordingly, it is possible that in the event of an IPO, the outstanding shares of common stock, common stock options, common stock
warrants and common stock issued in connection with the automatic conversion of the outstanding preferred stock and accrued dividends will, in aggregate, represent only 0.8% of the outstanding shares of our common stock on a fully-diluted basis
prior to the IPO and prior to giving effect to the expected grant of restricted stock units in connection with this offering under the Bonus Plan and Non-Employee Director Bonus Plan as more fully described in Notes 14 and 17 to our financial
statements appearing elsewhere in this prospectus. In order to effect such conversion rights, we will be required to issue to the holders of the 2009 Notes, 2010 Notes and 2011 Notes such number of shares of common stock as is required to effect
such conversion rights. In April 2012, our board of directors and stockholders declared a 1-for-5,670.66 reverse stock split that will be effected prior to issuing such number of shares of common stock necessary to effect the conversion of the 2009
Notes, 2010 Notes and 2011 Notes. This reverse stock split will materially impact both the number of shares of common stock and common stock equivalents outstanding, as well as the exercise price of such options and other common stock equivalents.

If we had made different assumptions and estimates, the fair value of our common stock and the amount of our stock-based
compensation expense could have been materially different. We believe that we have used reasonable approaches, methodologies and assumptions in determining the fair value of our common stock.

Fair Values of Change of Control and Liquidation Put Rights

Each of the 2009 Financing,
2010 Financing and 2011 Financing contains a provision, or put right, which provides that upon a change of control or liquidation, as defined in the agreements, the noteholders are entitled to an amount that is equal to the greater of: (i) the
sum of (y) 1.75x, 3.5x and 3.5x, respectively, of the principal amount plus (z) any accrued but unpaid interest and (ii) the amount the holder would be entitled to receive if the outstanding debt amount converted into shares of common
stock immediately prior to repayment. These put rights are considered derivative instruments that require liability classification and mark-to-market accounting at each balance sheet date.

Therefore, upon each convertible notes payable closing, we determined the fair values of the put rights using the PWERM valuation analysis method
consistent with the discussion above for the common stock valuations, and recorded the put rights as a liability on the balance sheets and as debt discount to be amortized to interest expense through the earliest date upon which we could have been
required to repay the amounts outstanding for each respective convertible notes payable issuance. The debt discount related to the initial fair values of the put rights

associated with the 2009 Notes was amortized to interest expense through January 8, 2010, which was the original earliest date at which we could have been required to repay all amounts
outstanding under the 2009 Notes. During the year ended December 31, 2010, the debt discount related to the initial fair values of the put rights associated with the 2010 Notes was amortized to interest expense through March 31, 2011,
which was the original earliest date at which we could have been required to repay all amounts outstanding under the 2010 Notes. Subsequent to the amendment of the 2010 Notes in connection with the January 2011 Financing, which amended the original
earliest date at which we could have been required to repay all amounts outstanding under the 2010 Notes from March 31, 2011 to June 30, 2012, the unamortized debt discount remaining at January 2011 was being amortized to interest expense
through June 30, 2012. The debt discount related to the initial fair values of the put rights associated with the 2011 Financing was being amortized to interest expense through June 30, 2012, which was the earliest date at which we could
have been required to repay all amounts outstanding under the 2011 Notes. Pursuant to a subordination agreement executed in connection with the February 2012 loan agreement, we determined that the change to the date on which the lenders can put the
debt back to us is a modification of the 2009 Notes, 2010 Notes and 2011 Notes. As such, the unamortized debt discount remaining at February 2012 related to the 2009 Notes, 2010 Notes and 2011 Notes will be amortized to interest expense from the
date of modification through June 1, 2015, the stated maturity date of the loan agreement.

At the end of each reporting period, the
fair values of the put rights are determined by management using the PWERM valuation analysis method consistent with the discussion above for the common stock valuations, and the recorded value adjusted, with any changes recorded as a component of
interest expense. We will continue to adjust the fair values of the put rights at each period end for changes in fair value until the conversion or repayment of the associated convertible notes payable.

Fair Values of Common Stock Warrants

As part of the 2009 Financing, 2010 Financing and 2011 Financing, we issued warrants for the purchase of common stock. The 2009 warrants include a
provision that provides for a reduction in the warrant exercise price if we subsequently issue additional shares of common stock for consideration per share less than the warrant exercise price. The 2010 and 2011 warrants include provisions that
provide for a reduction in the warrant exercise price and an increase in the number of exercisable warrants if we subsequently issue additional shares of common stock for consideration per share less than the warrant exercise prices. As a result,
the warrants have been deemed to be derivative instruments that require liability classification and mark-to-market accounting at each balance sheet date. Subsequent to the completion of an IPO, the provisions described above will no longer be
applicable, and as such the requirements for liability classification and mark-to-market adjustments will cease.

Upon issuance, we
estimated the fair values of these warrants using a multiple scenario probability-weighted option-pricing model with the following inputs: the estimated fair value of the underlying common stock at the valuation measurement date; the risk-free
interest rates; the expected dividend rates; the remaining contractual terms of the warrants; the expected volatility of the price of the underlying common stock; and the probability of various liquidity events. Our estimates are based, in part, on
subjective assumptions and could differ materially in the future. The fair values of these warrants were recorded as liabilities on the balance sheets and as debt discount to be amortized to interest expense through the earliest date upon which we
could have been required to repay the amounts outstanding for each respective convertible notes payable issuance. The debt discount related to the initial fair values of the warrants issued in connection with the 2009 Financing was amortized to
interest expense through January 8, 2010,

which was the original earliest date at which we could have been required to repay all amounts outstanding under the 2009 Notes. During the year ended December 31, 2010, the debt discount
related to the initial fair values of the warrants issued in connection with the 2010 Financing was amortized to interest expense through March 31, 2011, which was the original earliest date at which we could have been required to repay all
amounts outstanding under the 2010 Notes. As a result of the amendment of the 2010 Notes in connection with the 2011 Financing, which amended the original earliest date at which we could have been required to repay all amounts outstanding under the
2010 Notes from March 31, 2011 to June 30, 2012, the unamortized debt discount remaining at January 2011 was being amortized to interest expense through June 30, 2012. The debt discount related to the initial fair values of the
warrants issued in connection with the 2011 Financing was being amortized to interest expense through June 30, 2012, which was the earliest date at which we could have been required to repay all amounts outstanding under the 2011 Notes.
Pursuant to a subordination agreement executed in connection with the February 2012 loan agreement, we determined that the change to the date on which the lenders can put the debt back to us is a modification of the 2009 Notes, 2010 Notes and 2011
Notes. As such, the unamortized debt discount remaining at February 2012 related to the 2009 Notes, 2010 Notes and 2011 Notes will be amortized to interest expense from the date of modification through June 1, 2015, the stated maturity date of the
loan agreement.

At the end of each reporting period, the fair values of the warrants are determined by management using a multiple
scenario probability-weighted option-pricing model using the following inputs: the estimated fair value of the underlying common stock at the valuation measurement date; the risk-free interest rates; the expected dividend rates; the remaining
contractual terms of the warrants; the expected volatility of the price of the underlying common stock; and the probability of various liquidity events, with any changes in value during the period recorded as a component of interest expense. We will
continue to adjust the fair values of the warrants at each period end for changes in fair value until the earlier of the exercise or expiration of the applicable common stock warrants or the completion of this offering.

JOBS Act

On April 5, 2012, the
Jumpstart Our Business Startups Act of 2012, or the JOBS Act, was enacted. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period provided in Section 7(a)(2)(B)
of the Securities Act for complying with new or revised accounting standards. In other words, an emerging growth company can delay the adoption of certain accounting standards until those standards would otherwise apply to private
companies. We are electing to delay such adoption of new or revised accounting standards, and as a result, we may not comply with new or revised accounting standards on the relevant dates on which adoption of such standards is required for
non-emerging growth companies. Additionally, we are in the process of evaluating the benefits of relying on other exemptions and reduced reporting requirements provided by the JOBS Act.

Subject to certain conditions set forth in the JOBS Act, as an emerging growth company, we intend to rely on certain of these
exemptions, including without limitation, (i) providing an auditors attestation report on our system of internal controls over financial reporting pursuant to Section 404 and (ii) complying with any requirement that may be
adopted by the PCAOB regarding mandatory audit firm rotation or a supplement to the auditors report providing additional information about the audit and the financial statements (auditor discussion and analysis). These exemptions will apply
for a period of five years following the completion of our initial public offering or until we are no longer an emerging growth company, whichever is earlier.

The following table summarizes our revenues for the years ended December 31, 2010 and 2011:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2010

2011

(In thousands, except percentages)

Contract revenues

$



$

2,705

$

2,705

100

%

We did not record revenue during 2010. During 2011, we recognized $2.7 million of contract revenues under the
collaboration with Sanofi, based on the level of efforts expended from the inception of the collaboration with Sanofi in July 2011 through December 31, 2011.

Research and Development Expenses

The following table summarizes our research and
development expenses for the years ended December 31, 2010 and 2011:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2010

2011

(In thousands, except percentages)

Research and development expenses

$

12,422

$

31,206

$

18,784

151

%

Research and development expenses increased $18.8 million from 2010 to 2011. This increase was primarily due to an
$18.5 million increase in outsourced development costs in connection with delafloxacin, radezolid and our RX-04 program. During 2011, we initiated and completed enrollment in a Phase 2b clinical trial for delafloxacin for the treatment of ABSSSI to
evaluate the new Food and Drug Administration objective endpoint measurements and also conducted various Phase 1 clinical trials for delafloxacin in preparation for Phase 3 clinical trials anticipated to begin in the second half of 2012. During
2011, we completed a Phase 1 clinical trial for radezolid with an IV formulation and a long-term preclinical study of radezolid to further evaluate its safety. The overall $18.8 million increase in research and development expenses during 2011 as
compared to 2010 also included a $0.1 million decrease in personnel costs resulting from a slight decrease in headcount and a $0.4 million increase in travel and other costs.

General and Administrative Expenses

The following table summarizes our general and
administrative expenses for the years ended December 31, 2010 and 2011:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2010

2011

(In thousands, except percentages)

General and administrative expenses

$

5,152

$

5,723

$

571

11

%

The $0.6 million increase in general and administrative expenses from 2010 to 2011 was primarily due to: a $0.4
million decrease in personnel costs resulting from lower executive

recruitment and severance costs; a $0.8 million net increase in consulting costs related to increased legal, accounting and tax services, including higher legal costs incurred for the ongoing
pursuit of patent protection of our intellectual property, offset by lower business development and investor relation costs; and a $0.2 million increase in travel and other costs.

Interest Income

The
following table summarizes our interest income for the years ended December 31, 2010 and 2011:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2010

2011

(In thousands, except percentages)

Interest income

$

11

$

14

$

3

27

%

Interest income was essentially unchanged for 2011, as compared to 2010, due to the similar amounts of cash and
cash equivalents available for investment during the periods. During both 2010 and 2011, our interest income was not significant due to nominal cash and cash equivalent balances.

Interest Expense

The
following table summarizes our interest expense for the years ended December 31, 2010 and 2011:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2010

2011

(In thousands, except percentages)

Cash interest expense:

Notes payable

$

261

$



$

(261

)

(100

)%

Convertible notes payable

4,476

7,115

2,639

59

%

Sub-total cash interest expense

4,737

7,115

2,378

50

%

Non-cash interest expense:

Convertible notes payabledebt discount amortization

3,845

7,656

3,811

99

%

Convertible notes payablemark-to-market adjustments

1,407

4,392

2,985

212

%

Debt issuance costs amortization

246

334

88

36

%

Notes payable

55



(55

)

(100

)%

Sub-total non-cash interest expense

5,553

12,382

6,829

123

%

Total interest expense

$

10,290

$

19,497

$

9,207

89

%

The $9.2 million increase in interest expense from 2010 to 2011 was primarily due to: the increase of $2.6
million of cash interest expense resulting from the increase in outstanding convertible notes payable due to the 2011 Financing; an increase of $3.8 million in non-cash interest expense primarily resulting from debt discount amortization of the
initial fair values for the put rights in connection with the 2010 Financing and 2011 Financing; and an increase of $3.0 million in non-cash interest expense as a result of mark-to-market adjustments of the common stock warrants and put rights, as
further described above and under Fair Values of Change of

Control and Liquidation Put Rights and Fair Values of Common Stock Warrants. The increase in non-cash interest expense of $3.0 million resulting from mark-to-market
adjustments was primarily due to a $4.4 million aggregate increase in the fair values of the put rights in connection with the 2009, 2010 and 2011 Financings during 2011 as compared to a $2.5 million aggregate increase in 2010. Such increase in 2011
resulted from an increase in the estimated future enterprise values under the various sale/merger scenarios in the PWERM, based on significant changes in the business during the period. These changes included: signing an exclusive, worldwide
research collaboration and license agreement with Sanofi; completion of the delafloxacin Phase 2b clinical trial and receipt of top-line data in December 2011 allowing us to plan for the commencement of Phase 3 development in the second half of
2012; and the completion of a radezolid long-term preclinical study which showed favorable evidence of radezolids long-term safety profile. Such increases in estimated future enterprise values provided a greater allocation of value to such put
rights in the sale/merger scenarios offset by a decrease in the combined total probability from 35% to 30% for the sale/merger scenarios. In addition, during 2011, there was a de minimis negative adjustment to the fair value of the common stock
warrants whereas during 2010 there was a $1.1 million decrease in the fair values of such common stock warrants based on the decrease in the common stock value from December 31, 2009 to December 31, 2010. Such decrease in the fair values of
such common stock warrants was due to the dilutive effect of the rights and preferences of the put rights and common stock warrants issued in connection with the 2010 Financing. Accordingly, $1.1 million of the increase in interest expense in 2011
as compared to 2010 is attributable to the absence of the reduction in interest expense in 2010 resulting from the decrease in the fair value of such common stock warrants.

Other Income

The
following table summarizes our other income for the years ended December 31, 2010 and 2011:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2010

2011

(In thousands, except percentages)

Other income

$

1,098

$

246

$

(852

)

(78

)%

The $0.9 million decrease in other income from 2010 to 2011 was primarily the result of our receipt of $1.0
million in 2010 under the QTDP program. Other income for 2010 and 2011 included $0.1 million and $0.2 million, respectively, in connection with Connecticut research and development tax credit exchanges.

Comparison of the Years Ended December 31, 2009 and 2010

Revenues

We did not record revenue during the years ended December 31, 2009 and
2010.

Research and Development Expenses

The following table summarizes our research and development expenses for the years ended December 31, 2009 and 2010:

Research and development expenses decreased $5.2 million from 2009 to 2010. The decrease was primarily
due to a $4.3 million decrease in outsourced discovery and development costs in connection with delafloxacin, radezolid and our RX-04 program. This decrease was partially due to a $1.5 million milestone payment made to Wakunaga during 2009 for
delafloxacin with no corresponding payment during 2010, as well as a decrease in preclinical toxicology studies for delafloxacin. In connection with radezolid, we completed during 2009 a Phase 2 clinical trial for community acquired bacterial
pneumonia and there were no further costs incurred during 2010. In addition, there was a decrease in preclinical toxicology studies for radezolid. We increased our discovery activities for our RX-04 program by $0.2 million during 2010 as compared to
2009. The overall $5.2 million decrease in research and development expenses also included: a $1.1 million decrease in personnel costs resulting from a reduction in headcount affected in the second quarter of 2009 and lower stock-based compensation
costs in 2010 due to the decrease in our common stock value; offset by a $0.1 million increase in professional and consulting fees in connection with costs for European Union regulatory consulting services; and a $0.1 million increase in travel and
other costs.

General and Administrative Expenses

The following table summarizes our general and administrative expenses for the years ended December 31, 2009 and 2010:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2009

2010

(In thousands, except percentages)

General and administrative expenses

$

3,888

$

5,152

$

1,264

33

%

The $1.3 million increase in general and administrative expenses from 2009 to 2010 was primarily due to: a net
increase of $0.6 million in personnel costs including recruitment and severance costs incurred during 2010, offset by lower stock-based compensation costs in 2010 due to the decrease in our common stock value; a $0.5 million increase in consulting
costs related to additional accounting and tax services in 2010 as compared to 2009, human resources training efforts during 2010, additional business development and investor relation activities and legal fees during 2010 in connection with the
ongoing pursuit of patent protection of our intellectual property, offset by lower board of directors fees during 2010; and a $0.2 million increase in travel and other costs.

Interest Income

The
following table summarizes our interest income for the years ended December 31, 2009 and 2010:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2009

2010

(In thousands, except percentages)

Interest income

$

68

$

11

$

(57

)

(84

)%

The $0.1 million decrease in interest income for 2010 as compared to 2009 was the result of decreased cash and cash
equivalents balances available for investment. During both 2009 and 2010, our interest income was not significant due to nominal cash and cash equivalent balances.

The following table summarizes our interest expense for the years ended December 31, 2009 and 2010:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2009

2010

(In thousands, except percentages)

Cash interest expense:

Notes payable

$

1,055

$

261

$

(794

)

(75

)%

Convertible notes payable

2,591

4,476

1,885

73

%

Sub-total cash interest expense

3,646

4,737

1,091

30

%

Non-cash interest expense:

Convertible notes payabledebt discount amortization

7,831

3,845

(3,986

)

(51

)%

Convertible notes payablemark-to-market adjustments

(4,917

)

1,407

6,324

129

%

Debt issuance costs amortization

613

246

(367

)

(60

)%

Notes payable

(221

)

55

276

125

%

Sub-total non-cash interest expense

3,306

5,553

2,247

68

%

Total interest expense

$

6,952

$

10,290

$

3,338

48

%

The $3.3 million increase in interest expense in 2010 as compared to 2009 was primarily due to: an additional $1.9
million in cash interest expense during 2010 on the 2009 Notes and 2010 Notes; a reduction in non-cash interest expense of $4.0 million in connection with debt discount amortization as a result of the lower initial fair values for the put
rights and common stock warrants in connection with the 2010 Notes as compared to the 2009 Notes; and an increase of $6.3 million in non-cash interest expense as a result of the mark-to-market adjustments. The increase in non-cash interest expense
of $6.3 million resulting from mark-to-market adjustments was primarily due to a $4.2 million decrease in the fair values of the put rights in connection with the 2009 Financing during 2009 as compared to a $2.7 million increase in the fair value of
the put rights in connection with the 2010 Financing during 2010. The $4.2 million decrease in the fair value of the put rights in connection with the 2009 Financing during 2009 resulted from the 2010 Financing and related put rights which caused a
reduction in value of the put rights in connection with the 2009 Financing as the put rights in connection with the 2010 Financing have preference over the put rights in connection with the 2009 Financing. The $2.7 million increase in the fair
values of the put rights in connection with the 2010 Financing during 2010 resulted from the increase in the estimated future enterprise values under the various sale/merger scenarios from December 31, 2009 to December 31, 2010 due to positive
scientific progress for delafloxacin and the increased probability of completion of an RX-04 collaboration, which allowed for a greater allocation of value to the put rights in the sale/merger scenarios.

The following table summarizes our other income for the years ended December 31, 2009 and 2010:

Years EndedDecember 31,

Increase(Decrease)

%Increase(Decrease)

2009

2010

(In thousands, except percentages)

Other income

$

160

$

1,098

$

938

586

%

The $0.9 million increase in other income in 2010 as compared to 2009 was primarily the result of our receipt
of $1.0 million in 2010 under the QTDP program. Other income for 2009 and 2010 included $0.2 million and $0.1 million, respectively, in connection with Connecticut research and development tax credit exchanges.

Liquidity and Capital Resources

We have
incurred losses and negative cash flows from operations since our inception in October 2000, and as of December 31, 2011, we had an accumulated deficit of $244.3 million. We anticipate that we will continue to incur losses for at least the
next several years. We expect that our research and development and general and administrative expenses will continue to increase and, as a result, we will need additional capital to fund our operations, which we may obtain from public or private
equity, debt financings or other sources, such as corporate collaborations and licensing arrangements.

Since our inception in October
2000 through December 31, 2011, we have funded our operations principally through the receipt of $214.9 million from: the private placement of $122.4 million of preferred equity securities; the private placement of $71.0 million
of convertible notes payable; funds received under the collaboration with Sanofi of $19.0 million; and receipt of $2.5 million from research grants and government tax credit payments. As of December 31, 2011, we had outstanding
principal balances under our convertible notes payable of $71.0 million, accrued interest on convertible notes payable of $14.2 million and cash and cash equivalents of $8.0 million. Cash in excess of immediate requirements is
invested in accordance with our investment policy, primarily with a view to liquidity and capital preservation. Currently, our funds are held in cash and money market accounts.

Cash Flows

The following table sets forth the major sources and uses of cash and cash
equivalents for the years ended December 31, 2009, 2010 and 2011:

Years Ended December 31,

2009

2010

2011

(In thousands)

Net cash used in operating activities

$

(24,213

)

$

(16,746

)

$

(13,677

)

Net cash provided by (used in) investing activities

(750

)

626

(155

)

Net cash provided by financing activities

27,045

7,005

20,443

Net increase (decrease) in cash and cash equivalents

$

2,082

$

(9,115

)

$

6,611

Net cash used in operating activities. During 2009, 2010 and 2011, our operating activities used cash of
$24.2 million, $16.7 million and $13.7 million, respectively. The use of cash in all periods resulted from our net losses adjusted for non-cash items and changes in operating

assets and liabilities. The cash used in operating activities of $24.2 million in 2009 was primarily due to a net loss of $28.2 million primarily attributed to research and development activities
and interest expense, and decreases in accounts payable and accrued expenses of $2.1 million and $1.3 million, respectively, offset by non-cash interest expense of $3.3 million and an increase in accrued interest of $2.5 million. The cash used in
operating activities of $16.7 million in 2010 was primarily due to a net loss of $26.8 million primarily attributed to research and development activities and interest expense, offset by non-cash interest of $5.6 million and an increase in accrued
interest of $4.5 million. The cash used in operating activities of $13.7 million in 2011 was primarily due to a net loss of $53.5 million primarily attributed to research and development activities and interest expense, offset by non-cash
interest of $12.4 million and increases in accrued interest, accounts payable, accrued expenses and deferred revenue, in connection with our collaboration with Sanofi, of $7.1 million, $2.3 million, $0.8 million and
$16.3 million, respectively.

Net cash provided by (used in) investing activities. During 2009, our investing activities
used cash of $0.7 million, primarily attributable to purchases in excess of maturities of marketable securities. During 2010, our investing activities provided cash of $0.6 million, and were primarily attributable to maturities in excess of
purchases of marketable securities. During 2011, the cash used in investing activities of $0.2 million was a result of the purchase of fixed assets.

Net cash provided by financing activities. During 2009, 2010 and 2011, our financing activities provided net cash of $27.0 million, $7.0 million and $20.4 million, respectively. The cash provided by
financing activities of $27.0 million in 2009 was a result of the private placement of $34.9 million of convertible notes payable, net of debt issuance costs, offset by $7.9 million of repayments on borrowings under the credit and security
agreement. The net cash provided by financing activities of $7.0 million in 2010 was a result of the private placement of $14.2 million of convertible notes payable, net of debt issuance costs, offset by $7.2 million of repayments on borrowings
under the credit and security agreement. The credit and security agreement was paid in full during October 2010. The net cash provided by financing activities of $20.4 million in 2011 was a result of the private placement of $20.7 million
of convertible notes payable, net of debt issuance costs, offset by the payment of $0.2 million of deferred IPO costs.

Future Funding
Requirements

We believe that the net proceeds from this offering, together with the proceeds from the February 2012 loan
agreement, anticipated payments from the collaboration with Sanofi, and our existing cash and cash equivalents will be sufficient to fund our operations through the first quarter of 2014.

We anticipate that we will continue to incur net losses for the next several years due to expenses for our preclinical studies and clinical trials
including to:



commence two planned Phase 3 trials for delafloxacin for the treatment of ABSSSI in the second half of 2012;



initiate a Phase 2 study for radezolid in the treatment of ABSSSI and a long-term Phase 1 safety study for radezolid in humans to demonstrate what we believe is
a differentiable, long-term safety advantage over Zyvox;

The net proceeds from this offering will not be sufficient to fund our operations through the
successful development and commercialization of any of our product candidates. For example, to complete Phase 3 development of delafloxacin, we estimate that our two planned ABSSSI Phase 3 studies will each cost approximately
$33.0 million. As a result, we will need to raise additional capital beyond the proceeds of this offering to fund our operations and continue to conduct clinical trials to support potential regulatory approval of our product candidates,
including to commence our second planned Phase 3 trial of delafloxacin for the treatment of ABSSSI and to continue to advance the development of radezolid. To raise additional capital, we intend to seek funding through collaborations or other
similar arrangements with third parties. We may also seek to sell additional equity or debt securities, or incur indebtedness. The sale of additional equity and debt securities may result in additional dilution to our stockholders.

While the current terms of the 2009 Notes, 2010 Notes and 2011 Notes state that the lenders have the right to put the debt back to us on or after
June 30, 2012, which required that the 2009 Notes, 2010 Notes and 2011 Notes be classified as current in the December 31, 2011 balance sheet, we expect that such notes will instead be automatically converted into shares of common stock
immediately prior to the closing of this offering. However, pursuant to a subordination agreement executed in connection with the loan agreement, the holders of the 2009 Notes, 2010 Notes and 2011 Notes cannot demand or receive payment until such
time as all amounts due under the February 2012 loan agreement are paid in full in cash, and there is no further commitment on the part of the lender under the loan agreement to lend any further funds to us. Accordingly, with the exception of
interest payments totaling approximately $1.0 million due under the February 2012 loan agreement during 2012, we do not anticipate the need for any additional funds to service debt obligations during 2012, as the only outstanding notes payable
at December 31, 2011 were the convertible notes payable related to the 2009 Financing, 2010 Financing and 2011 Financing. We have based this estimate on the assumption that there are no events of default or a Material Adverse Change under the
February 2012 loan agreement, as well as other assumptions that may prove to be wrong, and we could utilize our available capital resources sooner than we currently expect. There can be no assurance that this offering will become effective or that
any securities will be sold pursuant to it. If we are unable to raise sufficient additional capital, we may need to substantially curtail our planned operations.

Our forecasts of the period of time through which our financial resources will be adequate to support our operations is a forward-looking statement and involves risks and uncertainties, and actual results could
vary as a result of a number of factors, including the factors discussed in Risk Factors.

Because of the numerous risks and
uncertainties associated with preclinical and clinical development, and given the early stage of our discovery programs, we are unable to estimate or know the nature of the efforts, specific timing or estimated costs that will be necessary to
complete the development of our product candidates. Our future funding requirements, both short-term and long-term, will depend on numerous forward-looking factors, including, but not limited to:

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the initiation, progress, timing, costs and results of preclinical studies and clinical trials for our product candidates and potential product candidates,
including initiation of Phase 3 development for delafloxacin;

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the success of our collaboration with Sanofi and receipt of milestones and royalty payments, if any, thereunder;

the amount and timing of any payments we may be required to make, or that we may receive, in connection with the licensing, filing, prosecution, defense and
enforcement of any patents or other intellectual property rights;

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the costs and timing of completion of commercial-scale outsourced manufacturing activities;

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the costs of establishing sales, marketing and distribution capabilities for any product candidates for which we may receive regulatory approval;

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the timing, receipt and amount of any sales, or royalties on, our product candidates, if any; and

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the terms and timing of any future collaborative, licensing or other arrangements that we may establish.

Our cash and cash equivalent balances as of December 31, 2011, significant debt outstanding and recurring losses from operations raise
substantial doubt about our ability to continue as a going concern. As a result, our independent registered public accounting firm included an explanatory paragraph in its report on our financial statements as of and for the year ended
December 31, 2011 with respect to this uncertainty. However, our financial statements have been prepared assuming we will continue to operate as a going concern, which contemplates the realization of assets and the satisfaction of liabilities
in the normal course of business. If we became unable to continue as a going concern, we could be unable to continue operations and could have to liquidate our assets.

Contractual Obligations

The following table summarizes our contractual obligations as of
December 31, 2011 and the effect such obligations are expected to have on our liquidity and cash flow in future years. While the current terms of the 2009 Notes, 2010 Notes and 2011 Notes state that the lenders have the right to put the debt
back to us, we expect that such notes will instead be automatically converted into shares of common stock immediately prior to the closing of this offering.

The convertible notes payable and accrued interest on convertible notes payable relate to the 2009 Financing, 2010 Financing and 2011 Financing. The amended
terms of the 2009 Notes provide that unless previously converted or repaid, principal and interest shall become due and payable on the later of (i) January 8, 2014 and (ii) the 91st day following the earlier of
(y) January 10, 2016 and (z) the date of payment or conversion in full of the 2011 Notes. The amended terms of the 2009 Notes also provide that upon a vote by the holders of 2009 Notes representing 60% of the outstanding principal
amount of all 2009 Notes, the holders of the 2009 Notes can demand repayment on or after June 30, 2012, provided there are no 2011 Notes or 2010 Notes outstanding. The amended terms of the 2010 Notes

provide that unless previously converted or repaid, principal and interest shall become due and payable on the later of (i) May 28, 2015 and (ii) the 91st day following the earlier
of (y) January 10, 2016 and (z) the date of payment or conversion in full of the 2011 Notes. The amended terms of the 2010 Notes also provide that upon a vote by the holders of 2010 Notes representing a majority of the outstanding
principal amount of all 2010 Notes, the holders of the 2010 Notes can demand repayment on or after the earlier of (i) June 30, 2012, provided there are no 2011 Notes outstanding, and (ii) the date on which we enter into an exclusive
product licensing transaction providing at least $30.0 million in upfront net cash proceeds to us. The terms of the 2011 Notes provide that unless previously converted or repaid, principal and interest on the 2011 Notes shall become due and payable
five years from the date of issuance. The terms of the 2011 Notes also provide that upon a vote by the holders of 2011 Notes representing a majority of the outstanding principal amount of all 2011 Notes, the holders of the 2011 Notes can demand
repayment on or after the earlier of (i) June 30, 2012 and (ii) the date on which we enter into an exclusive product licensing transaction providing us with at least $30.0 million in upfront net cash proceeds. However, pursuant to a
subordination agreement executed in connection with the February 2012 loan agreement, the holders of the 2009 Notes, 2010 Notes and 2011 Notes cannot demand or receive payment until such time as all amounts due under the February 2012 loan agreement
are paid in full in cash, and there is no further commitment on the part of the lender under the loan agreement to lend any further funds to us. The amounts for the convertible notes payable and accrued interest on convertible notes payable in the
table above are classified based on the stated maturity dates, and therefore do not reflect the earlier put dates of June 30, 2012.

(3)

The amounts in the table above exclude the principal and interest payments due under the loan agreement entered into in February 2012.

The table above reflects only payment obligations that are fixed and determinable. We enter into contracts in the normal course of business with CROs for clinical
trials and clinical supply manufacturing, and with vendors for preclinical research studies, research supplies and other services and products for operating purposes. These contracts generally provide for termination on notice, and therefore we
believe that our non-cancelable obligations under these agreements are not material. Milestone payments and royalty payments under our license agreements are not included in the table above because we cannot, at this time, determine when or if the
events triggering the commencement of payment obligations will occur.

Off-Balance Sheet Arrangements

Since inception, we have not engaged in the use of off-balance sheet arrangements, such as structured finance, special purpose entities or variable
interest entities.

Recently Issued Accounting Pronouncements

In October 2009, the FASB issued ASU 2009-13. ASC Topic 605-25 previously required companies to allocate revenue based on the fair value of each deliverable even though such deliverables may not be sold
separately either by the company itself or other vendors. ASU 2009-13 eliminates (i) the residual method of revenue allocation and (ii) the requirement that all undelivered elements must have objective and reliable evidence of fair value
before a company can recognize the portion of the overall arrangement fee that is attributable to items that already have been delivered. Allocation of consideration is now based on managements best estimate of the selling price for an
undelivered item where there is no other means to determine the fair value of that undelivered item. This revised accounting standard was effective prospectively for

revenue arrangements entered into or materially modified in fiscal years beginning on or after June 15, 2010. We adopted this guidance as of January 1, 2011 and it was applicable to the
Sanofi collaboration and license agreement we entered into in 2011.

In April 2010, the FASB amended ASU 2010-17, which provides
guidance on the milestone method of revenue recognition for research or development arrangements. Under the amended ASU 2010-17, an entity can make an accounting policy election to recognize a payment that is contingent upon the achievement of a
substantive milestone in its entirety in the period in which the milestone is achieved. The milestone method is not required and is not the only acceptable method of revenue recognition for milestone payments. This amended standard was effective
prospectively for milestones achieved during annual and interim reporting periods beginning on or after June 15, 2010. Early application is permitted. We adopted this guidance as of January 1, 2011 and it was applicable to the Sanofi
collaboration and license agreement we entered into in 2011.

In May 2011, the FASB issued ASU No. 2011-04, Fair Value
Measurement: Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in U.S. GAAP and IFRSs (ASC Topic 820). This newly issued accounting standard clarifies the application of certain existing fair value measurement
guidance and expands the disclosures for fair value measurements that are estimated using significant unobservable (Level 3) inputs. This ASU is effective on a prospective basis for annual and interim reporting periods beginning on or
after December 15, 2011. Early application is not permitted. We will adopt this amended guidance for the fiscal year beginning January 1, 2012.

Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on our
financial statements upon adoption.

Quantitative and Qualitative Disclosures about Market Risk

Our cash and cash equivalents as of December 31, 2011 consisted primarily of cash and money market accounts. Our primary exposure to market
risk is interest income sensitivity, which is affected by changes in the general level of United States interest rates. However, because of the short-term nature of the instruments in our portfolio, a sudden change in market interest rates
would not be expected to have a material impact on our financial condition and/or results of operation. The interest rates on our convertible notes payable are fixed and therefore not subject to interest rate risk. We do not have any foreign
currency or other derivative financial instruments.

We are a biopharmaceutical company developing new antibiotics to provide superior
coverage, safety and convenience for the treatment of serious and life-threatening infections. Our proprietary drug discovery platform, which is based on Nobel Prize-winning science, provides an atomic-level, three-dimensional understanding of
interactions between drug candidates and their bacterial targets and enables us to systematically engineer antibiotics with enhanced characteristics. Our most advanced product candidate, delafloxacin, is intended for use as an effective and
convenient first-line therapy primarily in hospitals prior to the availability of a specific diagnosis. Unlike currently available first-line treatments, delafloxacin has the potential to offer broad-spectrum coverage as a monotherapy for serious
Gram-negative and Gram-positive bacterial infections, including for methicillin-resistant Staphylococcus aureus, or MRSA, with both intravenous and oral formulations. Delafloxacin has completed four Phase 2 clinical trials, including a Phase
2b clinical trial for the treatment of acute bacterial skin and skin structure infections, or ABSSSI. We have received results from this Phase 2b trial and plan to commence the first of two planned Phase 3 trials for the treatment of
ABSSSI in the second half of 2012. The timing of our second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this contemplated offering. Based on our current expectations regarding the availability
of such funding and subject to the results of these two trials, we anticipate submitting a New Drug Application for delafloxacin for the treatment of ABSSSI as early as the fourth quarter of 2014 and for additional indications thereafter. Our second
product candidate, radezolid, is a next-generation, IV/oral oxazolidinone designed to be a potent antibiotic with a safety profile permitting long-term treatment of resistant infections, including those caused by MRSA. We have completed two Phase 2
clinical trials of radezolid. We are also pursuing development of RX-04, our preclinical program partnered with Sanofi, S.A., which has produced new classes of antibiotics that attach to a location on the bacterial ribosome to which no other
approved class of antibiotics bind, and are designed to combat the most difficult-to-treat, multi-drug resistant Gram-positive and Gram-negative bacteria. Because its protein building function is essential for the life of infection-causing bacteria,
the bacterial ribosome is the target of most marketed antibiotics, which work by binding to the ribosome and inhibiting its function. In addition, our pipeline includes RX-05, an antibacterial discovery program, and RX-06, an antifungal discovery
program, both of which target newly discovered binding sites within ribosomes.

We believe one of our key competitive advantages is our
focus on the three-dimensional properties of antibiotics, which is enabled by our proprietary drug discovery platform. Unlike traditional approaches to antibiotic discovery, which generally rely on random screening of chemical libraries to identify
potential compounds, our discovery team utilizes sophisticated, customized computer software to simulate and predict in three-dimensions both inter- and intra-molecular reactions and resulting properties of compounds including absorption,
distribution, metabolism, excretion and toxicology. We combine these exclusive computational tools with our patent-protected, atomic-level insights into the structure of the ribosome to systematically engineer novel antibiotics to avoid resistance
and optimize potency, spectrum, efficacy and safety. As a result, we have created a highly efficient and productive drug development engine based on our unique design strategy that effectively leverages structure-based drug design, preparative
medicinal chemistry, ribosome biochemistry, molecular biology and pharmacology.

The growing issue of antibiotic-resistant bacterial infections has been
widely recognized as an increasingly urgent public health threat, including by the World Health Organization, the Centers for Disease Control, or CDC, and the Infectious Disease Society of America, or IDSA. Antibiotic resistance has limited the
effectiveness of existing drugs, and the discovery of new antibiotics to deal with resistance has not kept pace with the increasing incidence of difficult-to-treat microorganisms. Staphylococcus skin and soft tissue infections in the
United States alone accounted for on average nearly 12 million physician and emergency department visits annually in the years from 2001 to 2003 according to the CDC. In addition, the IDSA estimated in 2004 that nearly two million
infections are developed in the hospital setting annually in the United States, resulting in the deaths of 90,000 patients each year. Of these infections, 70% are caused by bacteria that are resistant to one or more antibiotics used to treat them,
including those caused by MRSA. The CDC estimated that MRSA alone caused 94,000 life-threatening infections and almost 19,000 deaths in 2005 in the United States, exceeding the number of deaths caused by HIV/AIDS during the same time period. We
estimate that the use of antibiotics to treat MRSA has increased at a compounded annual growth rate of 18% for the years from 2005 to 2009 and is forecasted to continue growing through 2019. In April 2011, IDSA issued a report published in
Clinical Infectious Diseases warning that unless significant measures are taken now to increase the pipeline of new antibiotics active against drug resistant infections, people will start to die from common, formerly treatable infections, and
medical interventions such as surgery, chemotherapy, organ transplantation and care of premature infants will become increasingly risky.

Bacterial infections are caused by a variety of different types of bacteria that attack the body as they enter through the skin, lungs, nasal
passages or gastrointestinal tract, and are not controlled by the bodys immune system. They can range from mild to serious, life threatening infections requiring immediate treatment. Bacteria are broadly categorized as Gram-positive,
Gram-negative, atypical or anaerobic. Gram-positive bacteria possess a single membrane and a thick cell wall and turn dark-blue or violet when subjected to a laboratory staining method known as Grams method. Common causes of Gram-positive
bacterial infections include species of Staphylococcus, such as MRSA, Streptococcus and Enterococcus. Gram-negative bacteria have two membranes with a thin cell wall and, when subjected to Grams method of staining, lose
the stain or are decolorized. According to The New England Journal of Medicine, the most common cause of Gram-negative infection is Escherichia coli, or E. coli. Less prevalent Gram-negative bacteria strains include species of
Acinetobacter, Enterobacter and Pseudomonas. Atypical bacteria, such as Legionella species, have modified cell walls and are neither Gram-positive nor Gram-negative. Anaerobic bacteria, such as Bacteroides species,
either cannot grow in the presence of oxygen or do not require oxygen to grow.

According to the CDC, many strains of bacteria have
mutated over time to develop resistance to existing drugs, resulting in increasingly serious and more difficult to treat infections, and the rates of infections caused by single- or multi-drug resistant microorganisms continue to rise globally.
Based on U.S. data from Clinical Infectious Diseases and Canadian data from Antimicrobial Agents and Chemotherapy, we estimate that approximately 30% of these infections contain multiple strains of bacteria, including Gram-positive and
Gram-negative strains. The CDC estimates that the extra cost to the U.S. health care system in 2009 from health care associated infections ranged between $36 and $45 billion annually after adjusting to 2007 dollars using the Consumer Price Index for
inpatient hospital services. According to the CDC, approximately 60% of certain common resistant infections acquired during hospital stays are caused by MRSA. Another resistant Gram-positive infection, vancomycin-resistant enterococci,

or VRE, has also become increasingly common. According to the CDC, from 1990 to 2003, the prevalence of VRE in enterococcal isolates from hospitalized patients increased from less than 1%
to approximately 29%. While the CDC has found that MRSA remains the single most common resistant bacterial infection, and MRSA and VRE together constitute a significant majority of resistant bacterial infections, resistant Gram-negative infections
are also rapidly increasing. For example, according to the CDC, in 1997, the SENTRY Antimicrobial Surveillance Program found that among Klebsiella pneumoniae strains isolated in the United States, resistance rates to ceftazidime and other
third-generation cephalosporins were 7%, 10%, 5%, and 4% for bloodstream, pneumonia, wound, and urinary tract infections, respectively. However, by 2003, 21% of all Klebsiella pneumoniae isolates from intensive care units included in the
National Nosocomial Infections Surveillance System were resistant to these drugs. In addition, Gram-negative strains of Acinetobacter, Enterobacter and Pseudomonas are particularly life threatening, having mortality rates,
according to Datamonitor, of 34%, 27% and 39%, respectively, as of 2004.

Antibiotics are also characterized by their basic molecular
structure, which is known as their antibiotic class. Of the more than 20 different classes of antibiotics, only two new classes of hospital antibiotics have been commercialized in the last 40 years. Antibiotics are primarily
differentiated based on their potency and effectiveness against particular strains of bacteria as well as the spectrum of bacterial strains against which they are active. For example, broad-spectrum antibiotics are active against both Gram-positive
and Gram-negative bacteria and narrow-spectrum antibiotics are usually active against only a select subset of Gram-positive or Gram-negative bacteria. Because it usually takes from 24 to 48 hours to definitively diagnose a particular bacterial
infection, effective first-line treatment in hospital emergency departments of serious infections requires the use of antibiotics with broad-spectrum coverage, including coverage of MRSA, until the bacterial infection can be diagnosed. In general,
while physicians generally prefer narrower spectrum antibiotics once a pathogen has been identified, they will typically continue with broad-spectrum treatment to conclusion if a patients infection is improving. Other important characteristics
in distinguishing among antibiotics include their safety and tolerability, the frequency and route of administration of their dosing and their likelihood of developing resistant bacterial strains.

Market Opportunity

According to
Datamonitor, in the seven major pharmaceutical markets, which consist of the United States, Japan, the United Kingdom, Germany, France, Italy and Spain, antibiotic product sales totaled approximately $20 billion in 2009 and, within the hospital
market, approximately $8 billion was generated from antibiotic sales in 2006. Based on data published by third party investment research providers, in 2009, the global market for products targeting MRSA was approximately $2.6 billion and the
total Staphylococcal market was approximately $3 billion. Based on data from public filings with the Securities and Exchange Commission, as well as data from third party investment research providers, we estimate that in 2011, sales in the
United States of major antibiotics, such as vancomycin, daptomycin, marketed as Cubicin, and linezolid, marketed as Zyvox, which are designed to treat serious infections caused by resistant Gram-positive bacteria like MRSA, amounted to approximately
$1.6 billion. Based on data provided by GlobalData for the U.S. pharmaceutical market and the global pharmaceutical market, we estimate that the use of antibiotics to treat MRSA has increased at a compounded annual growth rate of 18% for the years
between 2005 and 2010 and is forecasted to continue growing through 2017. There have been no new classes of antibiotics approved with broad-spectrum coverage or to treat multi-drug resistant Gram-negative pathogens in the last 20 years.

The most widely prescribed antibiotics currently used by hospitals to treat multi-drug resistant
infections, such as MRSA and Gram-negative bacteria, include:

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Vancomycin. The current first-line standard of care in hospital emergency rooms for serious infections is an antibiotic cocktail consisting of
vancomycin, a generic intravenous, or IV, therapy, and a Gram-negative therapy. Vancomycin is also used as focused therapy for certain Gram-positive infections. According to GlobalData, in 2010, vancomycin generated sales of approximately $144
million in the United States and represented the majority of courses prescribed. Vancomycin is associated with allergic reactions and can cause kidney damage, or renal toxicity, loss of balance, or vestibular toxicity, and loss of hearing, or
oto-toxicity in certain patients. The rise of vancomycin-resistant bacterial strains in recent years, such as VRE, vancomycin intermediate-resistant Staphylococcus aureus, or VISA, and vancomycin-resistant Staphylococcusaureus,
or VRSA, has also reduced the drugs usefulness. These limitations of vancomycin are increasingly requiring a second-line treatment such as Zyvox (linezolid) or Cubicin (daptomycin) and additional days spent in the hospital.

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Zyvox. Zyvox (linezolid), a twice-per-day IV/oral oxazolidinone, is a leading antibiotic for serious Gram-positive infections, including MRSA. It
is used predominantly as a second- or third-line treatment and as a branded treatment alternative to vancomycin for resistant Gram-positive infections. In 2011, Pfizer Inc. reported Zyvox sales of $1.3 billion worldwide, including $640 million in
the United States. Zyvox requires active monitoring for use beyond two weeks of therapy due to the potential for significant adverse side effects, including bone marrow suppression, or myelosuppression, and nerve damage, or neurotoxicity. Zyvox use
has been associated with drug resistance, including increased incidence of Zyvox-resistant Staphylococcus aureus, or ZRSA, and Zyvox-resistant enterococci, or ZRE. Zyvox is also a narrow-spectrum drug, which prevents its use as a
monotherapy for the first-line treatment of serious bacterial infections. In addition, Zyvox is contraindicated for use in people taking monoamine oxidase inhibitors, a class of drugs formerly commonly used as anti-depressants, and should not be
used without careful observation in people taking selective serotonin reuptake inhibitors, a class of drugs commonly used as anti-depressants, among other uses. We believe that these issues will continue to limit the use of Zyvox, as well as any
generic form of linezolid available in the future.

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Cubicin. Cubicin (daptomycin), a lipopeptide, is another common antibiotic used as a second-line treatment and as a branded treatment alternative
to vancomycin for infections due to resistant Gram-positive bacteria, particularly MRSA. In 2011, Cubist Pharmaceuticals, Inc. reported net revenues in the United States of $699 million for Cubicin. Cubicin is only available in an IV form and
requires regular laboratory monitoring for toxic side effects in certain patients. Cubicin is also not indicated for the treatment of lung infections. In addition, bacterial resistance to Cubicin is increasing.

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Tygacil. Tygacil (tigecycline), an IV-only glycylcycline, a sub-class of tetracyclines, was approved by the FDA in June 2005. Tygacil is a
broad-spectrum antibiotic with coverage of resistant Gram-positive bacteria, such as MRSA, and Gram-negative bacteria, such as E. coli. Tygacil is generally utilized as a third- or fourth-line antibiotic due to its greater risk of
mortality as compared to the active comparators in its clinical studies and high rates of vomiting and nausea. According to Pfizer, Tygacil generated sales of $298 million globally, including $148 million in the United States in 2011.

Our objective is to discover, develop and commercialize best-in-class and new classes of anti-infectives with superior coverage, safety and convenience to provide new standards of care for patients with serious and
life-threatening infections. The critical components of our business strategy are:

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Complete the clinical development of delafloxacin. We plan to commence the first of two planned Phase 3 trials for the treatment of ABSSSI in the
second half of 2012. The timing of our second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this contemplated offering. Based on our current expectations regarding the availability of such
funding and subject to the results of these two trials, we anticipate submitting applications for marketing approval to the U.S. Food and Drug Administration, or FDA, and European Medicines Agency, or EMA, as early as the fourth quarter of 2014. We
also intend to seek approval for additional indications for delafloxacin, including CABP and cIAI.

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Opportunistically seek partners for the development and commercialization of our drug candidates outside of the United States. We have retained
worldwide rights to our drug discovery platform and all of our drug discovery and development programs other than the RX-04 program, where we maintain U.S. co-commercialization rights for one product candidate of our choosing. Outside of the United
States, we expect to seek strategic partnerships for the further development and commercialization of our product candidates, including delafloxacin and radezolid. We also intend to explore additional funded collaborations leveraging our drug
discovery platform.

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Advance the development of multiple product candidates from our RX-04 program through our collaboration with Sanofi. We intend to work with Sanofi
under our collaboration agreement to identify and develop multiple RX-04 product candidates. In addition to the development and commercial milestone payments for which we are eligible for each RX-04 product candidate, we intend to exercise our right
to co-commercialize one RX-04 product of our choosing in the United States. We expect that the product candidates that emerge from the RX-04 program will target a variety of uses, including the treatment of the most deadly and difficult-to-treat,
multi-drug resistant Gram-positive and Gram-negative pathogens.

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Leverage our discovery platform to continue to expand our pipeline of anti-infective product candidates. We intend to continue to pursue active
discovery programs using our proprietary platform to identify new binding sites within the ribosome and additional product candidates with broad-spectrum efficacy and safety to combat resistance mechanisms. In particular, we intend to demonstrate
evidence of potency, enabling lead identification and optimization in our RX-05 antibiotic program and our RX-06 antifungal program in 2012.

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Advance the clinical development of radezolid. We have successfully completed Phase 2 studies with an oral formulation of radezolid in
uncomplicated skin and skin structure infections, or uSSSI, and in CABP. Subject to obtaining sufficient additional funding beyond the proceeds of this contemplated offering, we intend to initiate a Phase 2 study for the treatment of ABSSSI and a
Phase 1 long-term safety study in humans to demonstrate what we believe is a long-term safety advantage over Zyvox. Following these studies, we also intend to perform additional clinical trials of radezolid in ABSSSI and CABP and for indications
that require long-term treatment, such as osteomyelitis and prosthetic and joint infections, including as a result of orthopedic surgery.

Our unique drug discovery approach serves as the foundation for our pipeline of clinical and earlier-stage product candidates, set forth below, that we believe can address the significant unmet needs for new
antibiotics that represent new treatment paradigms.

Delafloxacin

Our most advanced product candidate, delafloxacin, is intended for use as an effective and convenient first-line antibiotic primarily in hospitals prior to the availability of a specific diagnosis. Unlike current
first-line treatments, delafloxacin has the potential to offer broad-spectrum coverage as a monotherapy, including for MRSA, with both IV and oral formulations. The IV-to-oral switch offered by delafloxacin should provide patient convenience and
pharmacoeconomic benefits for maintaining this monotherapy. With the exception of Zyvox, all other currently approved treatments for MRSA offer only IV delivery. Delafloxacin has been in four Phase 2 trials where it has shown promising results for
the treatment of lung infections, including pneumonia and bronchitis, and skin infections. We believe that delafloxacin could treat many serious infections successfully and thus reduce the need for a switch to a narrow-spectrum therapy, because
physicians are less likely to switch if the broad-spectrum treatment is proving effective.

We have received results from our Phase 2b
clinical trial of delafloxacin for the treatment of ABSSSI, including infections caused by MRSA. This study, comparing delafloxacin to each of Zyvox and vancomycin, enrolled 256 subjects with ABSSSI. We expect to commence the first of

two planned Phase 3 clinical trials of delafloxacin for the treatment of ABSSSI in the second half of 2012 under a protocol that we intend to submit to the FDA for evaluation under the Special
Protocol Assessment, or SPA, process, and also to the EMA. We expect that both planned Phase 3 clinical trials of delafloxacin for the treatment of ABSSSI will be comparisons of delafloxacin against vancomycin in combination with a Gram-negative
therapy, and will also include a pharmacoeconomic evaluation. The timing of our second planned Phase 3 clinical trial will depend upon obtaining additional funding beyond the proceeds of this contemplated offering. Based on our current expectations
regarding the availability of such funding and subject to the results of these two trials, we anticipate submitting a New Drug Application for ABSSSI as early as the fourth quarter of 2014 and applications for additional indications thereafter.

Delafloxacin has been shown in in vitro testing to be more potent against MRSA than the current standard of care,
first-line MRSA treatment, vancomycin, and comparable in potency to Zyvox, Cubicin and Tygacil, the current leading second-line treatments for MRSA. Based on data from public filings with the Securities and Exchange Commission, as well as data from
third party investment research providers, total combined U.S. sales of Zyvox, Cubicin, vancomycin and Tygacil were approximately $1.6 billion in 2011. These compounds are predominantly used to treat MRSA or ABSSSI. Vancomycin is the most
commonly used of any of these drugs and is prescribed as a first-line treatment for Staphylococcal infections or as part of a broad-spectrum cocktail when combined with a Gram-negative focused compound such as cefepime.

Over 1,300 subjects have received delafloxacin in clinical trials to date and it has been generally well tolerated. In studies to date, we have
seen no myelosuppression, such as reported with the use of Zyvox, nor have we seen any adverse effects on muscles, which has been reported with the use of Cubicin and has resulted in required weekly monitoring of all patients taking Cubicin. In
addition, delafloxacin targets with equal potency two key enzymes found within bacteria, thereby decreasing the probability of resistance. These attributes, combined with delafloxacins potential as a monotherapy and opportunity for a
convenient IV-to-oral switch, provide a significant opportunity for widespread use of delafloxacin as a cost-effective, first-line treatment of serious Gram-positive and Gram-negative infections primarily in hospitals.

Differentiating Attributes

Key attributes differentiating delafloxacin from currently available antibiotics include:

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Increased potency at infection site. Delafloxacins unique three-dimensional structure and electronic charge distribution enables enhanced potency in
acidic environments, such as are common at the site of infection. Our in vitro testing suggests that delafloxacin becomes up to 32 times more potent in acidic environments than in normal pH environments within the human body, in contrast to
many other antibiotics which become progressively less potent, as measured by the changes in their minimum inhibitory concentration, or MIC, a measure of antibacterial potency. Moreover, delafloxacin kills bacteria more rapidly than Zyvox, Tygacil
and vancomycin according to the Journal of Chemotherapy and the Journal of Antimicrobial Chemotherapy.

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Expanded spectrum agent. In addition to strong Gram-positive potency, including against MRSA, delafloxacin has shown excellent in vitro activity
against most Gram-negative bacteria commonly found in the hospital setting, as well as against atypical and anaerobic pathogens. As a result of this expanded broad-spectrum coverage, delafloxacin has the potential to be used as a first-line
monotherapy for ABSSSI, bacterial pneumonias, aspiration pneumonias, complicated intra-abdominal infections, or cIAI, and complicated urinary tract infections.

Favorable safety profile. Over 1,300 subjects have received delafloxacin in clinical trials to date and it has been generally well tolerated. The common
adverse events observed in clinical trials of delafloxacin were nausea, diarrhea, vomiting, pruritus, fatigue, headache, dizziness, infusion site pain, insomnia, constipation, rhinitis and dry mouth. In addition, delafloxacin has not shown a
propensity for the toxicities that have been common in the fluoroquinolone class of antibiotics, such as sensitivity to light, or phototoxicity, impaired glucose regulation, or dysglycemia, and QT prolongation, a condition that may lead to or cause
heart rhythm irregularities. In a Phase I study, unlike other approved fluoroquinolones, delafloxacin showed no propensity to stimulate alterations in the QT interval. Further, delafloxacin has not exhibited the significant side effects associated
with the leading MRSA treatments. Active monitoring of Zyvox usage is required for use beyond two weeks of therapy due to the potential for adverse events such as myelosuppression and neurotoxicity. Vancomycins usage is limited by its side
effect profile, including renal toxicity, vestibular toxicity, oto-toxicity and allergic reactions. Cubicins potential to cause adverse muscle effects requires weekly monitoring in all patients of creatine phosphokinase, or CPK, an enzyme that
leaks out of damaged muscle cells. Tygacils side effect profile includes nausea, vomiting and an increased risk of mortality observed across all clinical studies.



Convenient IV-to-oral switch. We are developing both IV and oral formulations of delafloxacin to enable patients who begin IV treatment in the hospital
setting to transition to oral dosing for more convenient home-based care. We refer to this transition from IV treatment to oral dosing, which has the potential to lower the overall cost of treatment and reduce the length of hospital stays, as the
IV-to-oral switch. We believe this IV-to-oral switch has the potential to increase patient convenience, lower the overall cost of treatment and reduce the length of hospital stays.



Lower probability of bacterial resistance. Delafloxacin equally targets two enzymes found within bacteria, known as topoisomerase IV and DNA gyrase. Our
in vitro testing has shown that this ability to target both of these enzymes requires multiple mutations in both enzymes to produce resistance, which we believe reduces the probability of resistance to delafloxacin. In clinical trials to
date, we have not seen the development of bacterial resistance to delafloxacin.

The common adverse events observed in
clinical trials of delafloxacin were nausea, diarrhea, vomiting, pruritus, fatigue, headache, dizziness, infusion site pain, insomnia, constipation, rhinitis and dry mouth. Three patients receiving delafloxacin in our clinical trials have had
serious adverse events that were thought by the investigator to be possibly related to delafloxacin therapy. One patient with a previously non-disclosed recent onset seizure disorder had a further seizure on delafloxacin. One patient with a
complicated medical history was hospitalized with abdominal pain and diarrhea. A third patient had a single episode of mouth swelling and shortness of breath.

Our Phase 2b Trial in ABSSSI

We have received results from our Phase 2b clinical trial
designed to compare the efficacy of delafloxacin for the treatment of ABSSSI, including infections caused by MRSA to Zyvox (linezolid), with and without aztreonam, and vancomycin, with and without aztreonam. Delafloxacin met primary and secondary
efficacy endpoints evaluated to date, including endpoints based on the draft guidance from the FDA in ABSSSI. Of note, although this Phase 2b trial was not designed to demonstrate statistical significance, for the primary endpoint of
Investigators Global Assessment of Cure, delafloxacin demonstrated a statistically significant efficacy advantage as compared to vancomycin (95% Confidence Interval -30.3%, -2.3%;

p=0.031). Additionally, delafloxacin demonstrated numerical benefit over both Zyvox and vancomycin in the secondary endpoint, cessation of lesion spread and absence or resolution of fever at
48 to 72 hours, with cure rates of approximately 78%, 75%, and 73%, respectively. Furthermore, delafloxacin showed that a greater percentage of patients experience a 30% or greater reduction in the size of the lesion at 48 to 72 hours than either
comparator. Based on this analysis and other data, we believe delafloxacin has demonstrated a level of efficacy that strongly supports our planned initiation of a Phase 3 study of delafloxacin in the second half of 2012.

The differential between the cure rates of delafloxacin and vancomycin is statistically significant (95% Confidence Interval -30.3%, -2.3%; p=0.031).

(2)

ITTIntent-to-Treat

InvestigatorsGlobal Assessment of Cure in Patients with
Confirmed MRSA

Delafloxacin

Zyvox (withand withoutAztreonam)

Vancomycin (withand withoutAztreonam)

Response Rate

19/29

21/34

21/32

Percent Clinical Cure (MITT(1))

65.5

%

61.8

%

65.6

%

(1)

MITTMicrobiological Intent-to-Treat

Objective Endpoint at 48 to 72 hours(1)

Delafloxacin

Zyvox (withand withoutAztreonam)

Vancomycin (withand withoutAztreonam)

Response Rate

61/78

56/75

69/95

Percent Cessation of Spread of Erythema and Absence of Fever at 48 to 72 Hours

78.2

%

74.7

%

72.6

%

(1)

Objective efficacy measure proposed by FDA in Draft Guidance for Drug Development in ABSSSI in 2010.

Overall adverse event rates were statistically equivalent across the study for delafloxacin (74%), Zyvox (72%) and vancomycin (65%). The
leading adverse event associated with delafloxacin was gastrointestinal, or GI, disorder with mild to moderate diarrhea as the most common specific event. The other common adverse events in this trial were nausea, vomiting, fatigue, headache,
dizziness and infusion site pain. The leading adverse event for Zyvox was also GI disorder, with the most common specific event being nausea. The leading adverse event for vancomycin was disorders of the skin, with the most common specific event
being pruritus, or itching. In the Zyvox arm, two subjects experienced thrombocytopenia. In the vancomycin arm, three patients experienced renal issues, including two renal failures. Importantly, as observed in earlier Phase 2 studies, delafloxacin
did not demonstrate evidence for the toxicities that have been common in the fluoroquinolone class of antibiotics, such as phototoxicity, elevated liver enzymes, dysglycemia and QT prolongation.

The design of this trial grew out of extensive discussion with the FDA about new clinical trial
endpoints at our End-Of-Phase 2 meeting in April 2010, and subsequent discussions during the FDAs review of the trial protocol. Prior to the Phase 2b clinical trial, we had conducted a Phase 2a clinical trial the results of which supported our
decision to design and conduct a Phase 2b clinical trial. The Phase 2b study was a randomized, double-blind comparison of delafloxacin, Zyvox, with and without aztreonam, and vancomycin, with and without aztreonam, using objective efficacy measures
to evaluate the relative clinical responses in subjects with ABSSSI; aztreonam was added by the investigator based on the believed or confirmed presence of Gram-negative bacteria. The trial enrolled a total of 256 subjects across 34 centers in the
United States. Subjects were randomized into three treatment arms to receive either delafloxacin, 300 mg intravenously every 12 hours, or the recommended dosing for Zyvox (600 mg every 12 hours), both with and without aztreonam, or vancomycin (1,000
to 2,000 mg every 12 hours), both with and without aztreonam. The primary endpoint for the study was the Investigators Global Assessment of Cure. Additionally, a key goal was to assess the utility, variability and measurement techniques of
several objective measures of clinical efficacy for use in future clinical trials. Efficacy was evaluated at multiple time points during the study, with a focus on the first five days of administration, through assessments of objective signs and
symptoms of infection such as the extent/size of infection, fever, measurement of biochemical markers of inflammation and culture and susceptibility testing of bacterial isolates. Delafloxacin also demonstrated numerical benefit over both Zyvox and
vancomycin in the microbiological intent-to-treat population, a subset of the intent-to-treat population, with respect to the Investigators Global Assessment of Cure and the cessation of lesion spread and absence of fever at 48 and 72 hours.
The types of infections treated included abscess, wound, cellulitis and burn related infections. We believe that the application of these findings will enable us to optimize the design of our anticipated Phase 3 ABSSSI program.

Summary of Data from Previous Clinical Trials

Three previous Phase 2 studies and 16 Phase 1 studies of delafloxacin have been completed. One of the Phase 2 studies was completed with an IV formulation in complicated skin and skin structure infections, or
cSSSI, the former classification of ABSSSI, one was conducted with an oral formulation against community-acquired bacterial pneumonia, or CABP, and one was conducted with an oral formulation against acute bacterial exacerbation of chronic
bronchitis, or ABECB. Delafloxacin has been studied in more than 1,300 subjects to date. Across all completed studies, delafloxacin has been shown to be both clinically efficacious and well tolerated. Highlights of the clinical development of
delafloxacin to date include:



An IV formulation of delafloxacin in a Phase 2 clinical trial in cSSSI, in which delafloxacin was compared to tigecycline, showed that delafloxacin was better
tolerated with similar efficacy.



The oral formulation of delafloxacin has demonstrated excellent efficacy in once-daily dosing in Phase 2 clinical trials, for the treatment of CABP and ABECB.



Phase 1 studies show a low propensity for common fluoroquinolone toxicities such as phototoxicity, QT prolongation and dysglycemia, and no clinical adverse
events at the 300 mg dose related to these have been seen in Phase 2 studies of delafloxacin, indicating an excellent safety profile.

Phase 2 Study for Complicated Skin and Skin Structure Infections

We evaluated the IV
formulation of delafloxacin in a Phase 2 clinical trial for the treatment of cSSSI. This was a double-blind, multicenter, randomized study of IV delafloxacin compared with IV tigecycline for the treatment of cSSSI. In the trial, as compared to
tigecycline, delafloxacin was shown to be better tolerated with similar efficacy. This trial was not designed to demonstrate statistical significance and the results were not statistically significant.

One hundred and fifty male and female patients of at least 18 years of age were randomly assigned to
the treatment arms in a 1:1:1 ratio, to one of the following:



300 mg of delafloxacin every 12 hours;



450 mg of delafloxacin every 12 hours; or



tigecycline, in a dose of 100 mg initially, followed by 50 mg every 12 hours.

The duration of treatment was five to 14 days, subject to the investigators clinical judgment.

There were several analysis groups. The intent-to-treat, or ITT, group included all randomized patients who received at least one dose of drug. The
clinically evaluable, or CE, group was the subset of ITT patients who met key inclusion and exclusion criteria, did not receive confounding antibiotics for other infections, were present for key visits, and had key assessments performed. The
microbiologically evaluable, or ME, group were those patients that were clinically evaluable who also had a susceptible microbial pathogen identified at the screening visit, and met other specific criteria. The primary endpoint of this trial was the
investigators assessment of clinical response at the test-of-cure time point in the CE group. The test-of-cure time point was three to five days after the completion of dosing when investigators performed the global clinical assessment of
outcome and follow-up cultures for microbiology was obtained.

Response rates, as shown in the table below, were
numerically higher in the delafloxacin groups than in the tigecycline group in the ITT, CE, and ME populations. In addition, the eradication rates for baseline pathogen and for baseline MRSA for the ME population were higher in the delafloxacin
groups than in the tigecycline group. In the trial, 52% of all pathogens identified in the ME population were MRSA. In susceptibility testing performed on pathogens collected from patients in this study, delafloxacin had the lowest MIC90 of the 11 drugs tested. MIC90 values, which refer to the minimum concentration of a drug that will inhibit 90% of a pathogen in a particular population, are used
as a measure of the level of potential susceptibility of a pathogen to an antibiotic. A lower MIC90 number indicates greater potency.

Response Rates

Efficacy at Test Of Cure

Delafloxacin300 mg BID

Delafloxacin450 mg BID

Tigecycline100/50 mg BID

Clinical Response Rates

ITT Response

88% (43/49)

90% (46/51)

82% (41/50)

CE Response

94% (33/35)

93% (37/40)

91% (31/34)

Microbiological Response Rates

ME Response

97% (30/31)

94% (33/35)

89% (23/26)

The 300 mg dose of delafloxacin was well tolerated in this study and overall better tolerated than tigecycline.
None of the patients in the 300 mg group discontinued the study because of an adverse event. Two patients in the 450 mg group discontinued from the study due to adverse events. One patient with a previously undisclosed, untreated and ongoing seizure
disorder suffered a seizure during therapy and was discontinued. A second patient had moderately elevated liver enzymes which peaked at three to five times the upper limit of normal, and which returned to normal after discontinuation following ten
days of therapy. Bilirubin levels in the second patient remained normal. One patient in this trial had a serious adverse event that was thought by the investigator to be possibly related to delafloxacin therapy. This patient, who had a previously
non-disclosed recent onset seizure disorder, had a further seizure on delafloxacin. The common adverse events for delafloxacin in both 300 mg and 450 mg groups were nausea, diarrhea, fatigue, headache, insomnia and constipation.

A seven-day oral course of delafloxacin was tested in a double-blind, multicenter, randomized Phase 2 study to determine its safety and efficacy in
subjects with CABP. Three hundred nine subjects were randomized to one of three dosing groups: 100 mg, 200 mg or 400 mg delafloxacin once daily for seven days.

The analysis groups were defined as noted above. The primary endpoint of this trial was the investigators assessment of clinical response rate at the test-of-cure time point in the CE group. Clinical and
microbiological response rates are presented in the table below. All doses tested showed similar excellent response rates in all of the analysis groups. This trial was not designed to demonstrate statistical significance and the results were not
statistically significant. The common adverse events in this trial were nausea, diarrhea and headache.

Response Rates

Efficacy at Test of Cure

Delafloxacin100 mg

Delafloxacin200 mg

Delafloxacin400 mg

Clinical Response Rates

ITT

80% (83/104)

87% (79/91)

87% (90/104)

CE

90% (83/92)

95% (79/83)

93% (90/97)

Microbiological Response Rates

ME

90% (54/60)

96% (48/50)

94% (47/50)

Phase 2 Study for Acute Bacterial Exacerbation of Chronic Bronchitis.

A five-day oral course of delafloxacin was tested in a double-blind, multicenter, randomized, Phase 2 study to determine its safety and efficacy in
subjects with ABECB. The trial also included a treatment arm in which subjects received a seven-day course of levofloxacin tablets, to compare the results of that antibiotic to those of delafloxacin. Two hundred eighty subjects were randomized to
one of four dosing groups: 100 mg, 200 mg or 400 mg delafloxacin once daily for five days (QDx5 days) followed by two days of placebo, or 500 mg of levofloxacin once daily for seven days (QDx7 days).

The analysis groups were similar to those defined above. The primary endpoint of this trial was the investigators global assessment of
clinical response rate at the test-of-cure time point in the CE Group. Clinical response rates and microbiological response rates are presented in the table below. There was a trend toward better clinical and microbiological efficacy with the 200 mg
and 400 mg delafloxacin groups compared to the 100 mg delafloxacin group. While the once-daily 400 mg dosing level achieved comparable efficacy, we have no plans to continue with this dosing level. We have chosen to use a twice-daily IV 300 mg
dosing level, or an oral dose with an equivalent exposure, for our Phase 3 study. This trial was not designed to demonstrate statistical significance and the results were not statistically significant. Two patients in this trial had serious adverse
events that were thought by the investigator to be possibly related to delafloxacin therapy. One patient with a complicated medical history was hospitalized with abdominal pain and diarrhea. A second patient had a single episode of mouth swelling
and shortness of breath. The common adverse events for delafloxacin in this trial were diarrhea, headache, nausea, rhinitis, dry mouth, dizziness, insomnia, vomiting and sinusitis.

There have been a total of 16 Phase 1 studies conducted with delafloxacin to confirm safety or test various formulations of the drug. Key
safety-oriented Phase 1 studies include:



a randomized, double-blind, placebo- and positive-controlled study designed to evaluate the potential effects of delafloxacin on the QT interval of healthy adult
subjects. The QT interval is a measure of the length of time between electrical impulses in the heart, and is used to assess the potential for a drug to cause heart rhythm irregularities. This study was conducted according to the International
Committee for Harmonization, or ICH, Guideline 14 for the conduct of QT-effect studies. A total of 52 subjects, male and female, received the following regimens: placebo, moxifloxacin 400 mg oral as the positive control, delafloxacin 300 mg IV, and
the maximally tolerated delafloxacin dose of 900 mg IV. The data from this trial showed that delafloxacin did not prolong QT interval values at either the clinical dose of 300 mg IV or at the supra-therapeutic dose of 900 mg IV, and was similar to
placebo. The positive control, moxifloxacin, showed a typical prolongation response. No serious adverse events or heart rhythm irregularities were reported during the study. This study also confirmed the 900 mg dose as the maximum tolerated dose of
delafloxacin in an IV form.



a Phase 1, single-blind, placebo- and positive-controlled, randomized, parallel-group study in 52 healthy male and female volunteers. The study was designed to
demonstrate the photosensitizing potential and wavelength dependency of delafloxacin by comparing the response of the skin to ultraviolet A, or UVA, ultraviolet B, or UVB, and visible radiation prior to and during administration of delafloxacin,
lomefloxacin as the positive control, or placebo. Delafloxacin tablets were given in 200 mg and 400 mg once daily doses for six days and lomefloxacin was given in 400 mg once daily doses for six days. Delafloxacin was not different from the placebo,
and did not demonstrate clinically significant phototoxic potential at any of the wavelengths tested, while the active comparator lomefloxacin demonstrated a moderate degree of phototoxicity at UVA wavelengths of 335 nm and 365 nm.



a Phase 1 study of two IV formulations of delafloxacin, which included the collection of serum insulin values and corresponding serum glucose values in both the
300 mg and placebo groups on day one and day 14 of the study, beginning immediately pre-dose, followed by once per hour for six hours. The analysis showed no difference between placebo and delafloxacin in respect to insulin secretion and glucose
levels.

Radezolid

Our second product candidate, radezolid, is a next-generation, IV/oral oxazolidinone designed to meet the need for a potent antibiotic with a safety profile permitting long-term treatment of resistant infections,
including MRSA. While not yet tested for long-term use in humans, we have completed a 90-day animal toxicology study of radezolid which showed that radezolid was safely tolerated for the full 90 days at the maximum dose, which dose was 12 times
greater than the efficacious dose used in humans for shorter-term studies. Through a rational drug design process involving approximately 700 prototype compounds, we developed radezolid to have structural advantages that make it a candidate for use
as a treatment for serious infections, such as ABSSSI and severe CABP, and long-term treatment of underserved serious infections, such as osteomyelitis and prosthetic and joint infections. We believe that the demonstrated broad- spectrum of
coverage, potency and potential long-term safety profile of radezolid give it the potential to become the antibiotic of choice for multiple resistant bacterial infections, and for treatment in populations, such as the elderly and children, that
might be vulnerable to myelosuppression caused by other oxazolidinone treatments.

We have successfully completed two Phase 2 studies of radezolid with an oral formulation in uSSSI and
in CABP. In both studies, radezolid was shown to be well-tolerated and effective with a once-daily dosing regimen. We have also completed a Phase 1 study of an IV form of radezolid, to enable an IV- to-oral switch in future Phase 2 and Phase 3
studies. In in vitro studies, we have shown radezolid to be microbiologically more active than oxazolidinones currently on the market or known by us to be in development against Gram-positive microorganisms, including potent activity against
Zyvox-resistant bacteria. Subject to obtaining sufficient additional funding beyond the proceeds of this contemplated offering, we expect future clinical development of radezolid to involve an additional Phase 2 study in ABSSSI with the IV
formulation and a long-term Phase 1 safety study with the oral formulation. With its enhanced potency against resistant strains of MRSA, improved safety profile, and expanded spectrum, radezolid has the potential to be an improved oxazolidinone as
compared to Zyvox, the only currently marketed oxazolidinone and the leading branded antibiotic for serious Gram-positive infections, which generated worldwide sales of approximately $1.3 billion in 2011, as reported by Pfizer.

Differentiating Attributes

We believe that the structural advantages of radezolid confer the following attributes, which differentiate radezolid from Zyvox, tedizolid and other MRSA therapies and make it potentially suitable for the
treatment of serious infections, such as ABSSSI and severe CABP, including those caused by MRSA, as well as long-term treatment of underserved serious infections such as osteomyelitis, prosthetic and joint infections and tuberculosis:



Greater potency against Zyvox-resistant Gram-positive bacteria. Radezolid has several attributes allowing it to overcome known oxazolidinone resistance
mechanisms and have increased activity compared to oxazolidinones currently on the market or known by us to be in development. In a recent study presented in 2011 at the European Congress of Clinical Microbiology and Infectious Diseases, or ECCMID,
JMI Laboratories compared the potency of radezolid to that of the only marketed oxazolidinone, Zyvox, and another investigational oxazolidinone, tedizolid, against a collection of Gram-positive bacteria with genetically defined mechanisms of
oxazolidinone resistance. In this study, radezolid demonstrated enhanced activity against the collections 90 strains.



Broader spectrum. Like all oxazolidinones, radezolid has shown excellent in vitro activity against resistant Streptococcus pneumoniae and
MRSA. Unlike Zyvox and tedizolid, radezolid has also shown in vitro activity against other common causes of CABP, such as Haemophilus influenzae, Legionella pneumophila and Moraxella catarrhalis. In addition, radezolid
has shown potency and activity in animal models against Mycobacterium tuberculosis in a rodent study conducted by a third party.



Enhanced delivery to the site of infection. Our in vitro studies have shown that, unlike Zyvox, radezolid concentrates within cells, including
macrophages, other immune cells, and lung epithelial cells, and remains at concentrations and active within cells long after concentrations in the blood decrease following therapy withdrawal. These heightened cell concentrations enable enhanced
killing of intracellular pathogens and may convey greater drug quantities to infection sites through immune cell trafficking, making radezolid effective at lower and less frequent dosing.



Improved safety profile. In preclinical evaluations and clinical trials to date, radezolid administered at therapeutic doses has not demonstrated any
significant safety issues, such as the myelosuppression commonly associated with oxazolidinones, potentially making radezolid the only long-term oxazolidinone therapy available for difficult-to-treat MRSA and other resistant infections. The common
adverse events observed in clinical trials of radezolid were nausea, diarrhea, headache, dizziness and fungal infection. We

believe that, unlike Zyvox, radezolid does not enter the central nervous system. Therefore, it should not be subject to centrally-mediated effects due to monoamine oxidase inhibition and
serotonin interaction, including those often seen with Zyvox in patients taking selective serotonin reuptake inhibitors, or SSRI, agents such as Prozac. We recently completed a long-term preclinical study of radezolid to further evaluate its safety.
While the long-term safety of radezolid must be demonstrated in human clinical trials, the data from this preclinical study provided encouraging evidence of radezolids long-term safety profile.



Convenient IV-to-oral once daily dosing regimen. Radezolid can be administered in either IV or oral formulations. Moreover, unlike Zyvox, radezolid has
the potential to be administered once daily and at lower doses, which may improve patient compliance with the oral treatment regimen following discharge from the hospital and therefore increase the likelihood of treatment success.

The common adverse events observed in clinical trials of radezolid were nausea, diarrhea, headache, dizziness and
fungal infection. Two patients receiving radezolid in our clinical trials have had serious adverse events that were thought by the investigator to be possibly related to radezolid therapy. One patient with lung cancer had a pneumonia that did not
respond to radezolid therapy. A second patient with prior peptic ulcer disease discontinued ulcer therapy prior to enrolling in a radezolid trial and had a recurrent ulcer with perforation.

Our Phase 2 Clinical Trials of the Oral Dosage Form of Radezolid

Phase 2 Trial in Uncomplicated Skin and Skin Structure Infections. Our first Phase 2 study of radezolid was a multicenter, randomized,
open-label, comparative study to evaluate the safety and efficacy of radezolid as compared to Zyvox in the outpatient treatment of adult patients with uSSSI. A total of one hundred and fifty adult subjects with uSSSIs were randomized to one of three
treatment groups: radezolid 450 mg once per day, radezolid 450 mg twice per day, or Zyvox 600 mg twice per day. Patients took either radezolid or Zyvox orally for five days and were then clinically evaluated on study day five. If, upon such
evaluation, treatment for the uSSSI was still required, patients continued to take the study drug for up to an additional five days. ITT, CE, and ME analysis groups were as defined above. The primary endpoint of the study was clinical response at
test-of-cure time point. Of the pathogens isolated in the ME population at baseline, 54% were MRSA. The results of the trial were similar across all treatment groups and infection types. This trial was not designed to demonstrate statistical
significance and the results were not statistically significant. The common adverse events for radezolid in this trial were diarrhea and nausea. No episodes of bone marrow suppression with radezolid were noted. One patient on Zyvox had a temporary
reduction of platelets to below the lower limit of normal, while another had a temporary reduction of white blood count to below the lower limit of normal.

Response Rates

Efficacy at Test Of Cure

Radezolid450 mg QD

Radezolid450 mg BID

Zyvox600 mg BID

Clinical Response Rates

ITT Response

82% (40/49)

76% (37/49)

83% (39/47)

CE Response

97% (38/39)

94% (34/36)

97% (37/38)

Microbiological Response Rates

ME Response

100% (20/20)

89% (23/26)

91% (21/23)

Phase 2 Trial in Community-Acquired Bacterial Pneumonia. Our second Phase 2 study of radezolid was a
double-blind, randomized, multicenter clinical trial conducted in adult patients with mild to moderately severe CABP. The objectives of the study were to assess the efficacy, safety, and tolerability of radezolid in this patient population. We
enrolled and randomized a

total of 160 patients who met the study criteria into one of three radezolid treatment groups: 300 mg orally once per day, 450 mg orally once per day or 450 mg orally twice per day, in each
case for seven to 10 days. The primary endpoint of the study was clinicians assessment of clinical response at test-of-cure time point. This trial was not designed to demonstrate statistical significance and the results were not statistically
significant. Two patients in this trial had serious adverse events that were thought by the investigator to be possibly related to radezolid therapy. One patient with lung cancer had a pneumonia that did not respond to radezolid therapy. A second
patient with prior peptic ulcer disease discontinued ulcer therapy prior to enrolling in a radezolid trial and had a recurrent ulcer with perforation. The common adverse events in this trial were diarrhea, fungal infection, dizziness and headache.

Response Rates

Efficacy at Test Of Cure

Radezolid300 mg QD

Radezolid450 mg QD

Radezolid450 mgBID

Clinical Response Rates

ITT Response

80% (41/51)

80% (41/51)

70% (37/53)

CE Response

92% (34/37)

84% (37/44)

78% (32/41)

Microbiological Response Rates

ME Response

94% (16/17)

77% (20/26)

69% (18/26)

Other Clinical Studies

To date, we have tested the safety, tolerability, pharmacokinetics, food effect and age/gender effects of oral doses of radezolid in six Phase 1 studies. A total of 172 healthy volunteers were enrolled in these
studies and 143 received radezolid. Data from the six Phase 1 studies demonstrated that single oral doses of up to 1200 mg of radezolid and repeat oral doses of up to 900 mg of radezolid for 14 days were well tolerated in healthy volunteers. The
most common treatment-related adverse events experienced by the subjects in these trials were gastrointestinal symptoms such as diarrhea, loose stools and abdominal pain, together with headache, facial flushing and minor rash.

Three-month Rodent Toxicology Study

A three-month GLP rat toxicology study to support a future NDA submission was performed on our behalf by a third-party contract research organization, or CRO, to evaluate the long-term safety of radezolid dosed
orally. Radezolid was dosed in three arms of 10, 50 and 200 mg/kg/day, with a separate vehicle-dosed control group. The study was designed such that the mid-dose approximated the efficacious exposure in humans, and the higher dose, at four-times the
projected human efficacious exposure, was used to define an adverse event level. At the three-month time point, radezolid was found to be well-tolerated at all dose levels and resulted in no clinical observations, no changes in food consumption, and
minimal changes in body weight. Therefore, the CRO concluded that the no observed adverse effect level was 200 mg/kg/day. The absence of observed adverse events over 90 days, at a dosing regimen four-times greater than the projected efficacious dose
in humans, suggests that radezolid has strong potential as a long-term dosing option for resistant infections, although such potential must be confirmed through human clinical trials. The table below sets forth data with respect to dose, body
weight and changes in blood parameters for each dosing group.

In order to provide context for these results with respect to established findings for long-term
use of oxazolidinones as a class, the same CRO evaluated the long-term safety in rats of linezolid and tedizolid dosed orally over the same three-month period in a contemporaneous non-GLP study. For this study, linezolid, having 99.7% purity as
measured by high pressure liquid chromatography, or HPLC, was purchased from Alembic, Ltd., of India, and tedizolid, having 99.3% purity as measured by HPLC, was manufactured by a third party according to the procedure disclosed in the published
patent application (#WO 2010/042887 A2) of Trius. Linezolid was dosed in two arms of 40 and 100 mg/kg/day, and tedizolid was dosed in two arms of 10 and 50 mg/kg/day. The study was designed such that the lower doses approximated the efficacious
exposures in humans, with higher doses used to define adverse event levels. This study also had a separate vehicle-dosed control group. At the higher dose levels, mortality rates for both linezolid and tedizolid were significantly higher than for
comparably high doses of radezolid. Male and female rats in the high-dose linezolid group (100 mg/kg/day) exhibited significant weight loss, and all were taken off the study after 75 days pursuant to predetermined guidelines for the ethical
treatment of animals. Female rats in the high-dose tedizolid group (50 mg/kg/day) exhibited significant weight loss starting shortly after the commencement of dosing and, after 12 days of dosing at 50 mg/kg/day, were given a seven-day dosing
holiday, followed by a lower dose of tedizolid at 30 mg/kg/day. Due to significant weight loss, all of the female rats in the high-dose tedizolid group were taken off the study after 57 days, pursuant to the predetermined guidelines for the ethical
treatment of animals. The table below sets forth data with respect to dose, body weight and changes in blood parameters for each of the dosing groups.

Compound

Dose (mg/kg/day)

Average bodyweight at end ofstudy or grouptermination (Day*)

Hematology results at end of study orgroup termination (Day*)

Reticulocytes( x 103/µl)

Neutrophils( x 103/µl)

Control

-

580 M / 303 F

250.1 M / 190.2 F

1.71 M / 0.80 F

Linezolid

100

416 M / 213 F

(Day 71)

126.0 M / 175.1 F(Day 75)

0.56 M / 0.71 F(Day 75)

40

569 M / 318 F

(Day 75)

253.1 M / 218.9 F

1.52 M / 1.11 F

Tedizolid

50 (or 50/30 forfemales)

505 M (Day 91) /213 F (Day 57)

198.9 M (Day 91)/40.0 F (Day 58)

1.14 M (Day 91)/0.26 F (Day 58)

10

586 M / 304 F

217.9 M / 198.4 F

1.75 M / 0.82 F

*

Termination day, or measurement day, as applicable, if group did not complete 13 week study.

RX-04 Program

Our most advanced preclinical program, the RX-04 program is focused on
using a discrete, novel binding site within the ribosome to design and develop new classes of antibiotics to treat some of the most deadly and difficult-to-treat, multi-drug resistant Gram-positive and Gram-negative infections. Pathogens associated
with these infections include E.coli, Klebsiella pneumoniae, Enterobacter species, Pseudomonas aeruginosa and Acinetobacter baumannii and MRSA. Using our proprietary drug discovery platform, we have developed three novel classes
of antibiotics in less than three years that bind to this ribosome site, show high levels of antibacterial activity against a number of these pathogens, and show compelling efficacy in multiple animal models of infection. In June 2011, we entered
into a collaboration and license agreement with Sanofi to develop multiple products targeting this discrete binding site within the ribosome. See Strategic Collaboration with Sanofi.

Based on the preclinical data from the RX-04 program and on studies published in Antimicrobial
Agents and Chemotherapy, we believe that RX-04 is the only drug development program that has produced compounds with demonstrated in vitro coverage of all of the following multi- drug resistant Gram-positive bacteria: Enterococci,
Streptococci and Staphylococci, including MRSA; and multi-drug resistant and extremely drug resistant Gram-negative bacteria: Klebsiellapneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and E.
coli. These pathogens account for a majority of hospital-acquired infections, are associated with high rates of morbidity and mortality and are increasingly multi-drug resistant, meaning that they are resistant to more than three classes of
antibiotics. Because the RX-04 compounds bind to a novel site on the ribosome that has never before been exploited by marketed antibiotics and because they have proprietary chemical characteristics distinct from all current classes of broad-spectrum
or Gram-negative therapies, we have shown that their activity is unaffected by cross-resistance to current therapies, and we expect lower resistance development than current therapies. These compounds are also small molecules, thereby providing us
with the potential to ultimately offer an IV-to-oral switch for maximum flexibility. Furthermore, in vitro preclinical studies have shown that these compounds have little propensity for drug-drug interactions, have demonstrated no
cardiovascular toxicity in in vitro models, are not mutagenic and do not appear to have undesired pharmacological interactions. Based on these characteristics, we believe that the RX-04 program has significant potential to produce drug
candidates that directly address the urgent public health threat caused by the most difficult to treat pathogens.

Differentiating
Attributes

To call attention to the critical unmet need for new and better antibiotics, the IDSA established the 10x20
Initiative. The goal of the 10x20 Initiative is to build a sustainable antibiotic research and development infrastructure and, in the short-term, to produce 10 new systemic antibiotics by 2020 that target the ESKAPE pathogens, the six species
of drug-resistant bacteria that account for a majority of hospital-acquired infections, are associated with high rates of morbidity and mortality, and are increasingly multi-drug resistant, meaning that they are resistant to more than three classes
of antibiotics. They consist of the following Gram-positive bacteria, which provide the acronym ESKAPE: Enterococci and Staphylococcus aureus, including MRSA; and Gram-negative bacteria: Klebsiella pneumoniae, Acinetobacter
baumannii, Pseudomonas aeruginosa and Enterobacter species. Our RX-04 program demonstrates our commitment to this goal and, we believe, is well positioned to address the following desired attributes of these new antibacterial drugs:



Activity against extremely difficult-to-treat infections. We believe, based on published studies, that RX-04 may be the only drug development program that
has produced compounds with demonstrated in vitro coverage of all of the ESKAPE pathogens. Many of the ESKAPE pathogens are resistant to every antibiotic except colistin, a 1950s-era drug that is the treatment of last resort because of its
high toxicity.



Lower potential for resistance. Because the RX-04 compounds bind to a novel site on the ribosome that has never before been exploited by antibiotics and
because they have proprietary chemical characteristics distinct from all current classes of broad-spectrum or Gram-negative therapies, we expect that their activity will be unaffected by cross resistance to current therapies and will result in lower
potential resistance as compared to current therapies.



Lower potential for toxicity. Our discovery process allows optimization of molecular properties that have the potential not only for greater potency, but
also greater safety. We intend to use our compound-optimization abilities to design compounds which

Potential for IV-to-oral dosing. The RX-04 compounds are small molecules thereby providing us with the potential to develop an IV-to-oral switch for
maximum flexibility.

Summary of Preclinical Data

Several RX-04 compounds show compelling in vitro potency against panels of multi-drug resistant and extremely drug resistant ESKAPE pathogens
and have been unaffected by cross-resistance, including extended-spectrum beta-lactamase, or ESBL, Klebsiella pneumonia carbapenemase, or KPC, and New Delhi Metallo-beta-lactamase-1, or NDM-1, producing Enterobacteriaceae, multi-drug
resistant and extremely drug resistant A. baumannii and P. aeruginosa, MRSA, Vancomycin-resistant enterococci, or VRE, Zyvox-resistant enterococci, or ZRE, Macrolide-resistant streptococci and
staphylococci. Moreover, several compounds from this program have been shown to be efficacious when administered intravenously in mouse models of infection, including sepsis, skin and lung infections.

The table below compares the potency of two RX-04 program compounds, RX-7713 and RX-7999, to that of several marketed antibiotics against eight
representative multi-drug resistant and extremely drug resistant strains of bacteria. Potency is measured by MIC data, with lower numbers indicating greater potency. Using our proprietary drug discovery platform, we were able to design structural
improvements to RX-7713, which has excellent potency against many of these strains, resulting in RX-7999, which has excellent potency against all of these strains. This achievement was accomplished in six months and involved the design and testing
of only 67 analog compounds.

Several compounds from the RX-04 program showed efficacy in multiple animal models
of infection. Set forth in the table below are mouse survival data from two sepsis models of infection, one with MRSA and the other with Acinetobacter baumannii. The results are expressed as PD50s, which is the amount of drug needed to ensure survival in half of the study animals, with lower numbers indicating a higher level
of protection. In addition to these excellent survival results, RX-04 compounds have been shown to be highly efficacious in other models of animal infection, including an E. coli sepsis model, a Klebsiella pneumoniae,
skin-and-soft-tissue model, and a MRSA kidney abscess model.

Compound

MRSA 11540

A. baumannii1705943

MIC(mg/mL)

PD50(mg/kg)

MIC(mg/mL)

PD50(mg/kg)

RX-7892

0.5

<1.0

0.25

<0.5

RX-8082

0.125

<0.5

0.25

1.57

RX-8119

0.25

<0.5

0.25

0.99

RX-8120

0.25

<0.5

0.25

0.85

Strategic Collaboration with Sanofi

In June 2011, we entered into a collaboration and license agreement with Sanofi related to our RX-04 program. Under this agreement, Sanofi has the right to license an unlimited number of product candidates
targeting a discrete binding site within the ribosome. We retain all rights pertaining to our proprietary drug discovery platform, including all other binding sites within the ribosome and all future programs, as well as to any RX-04 compound that
Sanofi does not exercise its option to develop during the three-year term of the collaboration. We have received $22.0 million through March 31, 2012 in upfront and milestone payments under the collaboration, including the receipt of a payment of
$3.0 million from Sanofi in January 2012 for the achievement of a research milestone. For each RX-04 product developed by Sanofi, we are eligible for up to $9.0 million in potential research milestone payments, up to $27.0 million in potential
development milestone payments relating to initiation of Phase 1, 2 and 3 clinical trials, up to $50.0 million in potential regulatory milestone payments relating to approvals in various jurisdictions including the United States, the European Union
and Japan, up to $100.0 million in potential commercial milestone payments, and tiered percentage royalties of up to 10% on sales from products commercialized under the agreement, if any. We also have the right under the collaboration to
co-commercialize one RX-04 product of our choosing with Sanofi in the United States. We are currently collaborating with Sanofi on ongoing preclinical development and lead generation and, as part of a comprehensive safety assessment, we have just
completed in vitro and in vivo profiling of the first cohort of leads from the RX-04 program that demonstrated strong potency and efficacy. These results have informed the next iteration of design and optimization. We expect the
results of this optimization round to inform the selection of a lead compound in 2012 for toxicology studies followed by Phase 1 studies in humans. Either party may terminate this agreement immediately upon written notice if the other party becomes
subject to bankruptcy or similar events. In addition, either party may terminate this agreement upon 30 days written notice if the other party, or the other partys affiliates or sublicensees, challenges the validity or enforceability of
a claim included in any patent licensed to such other party, affiliate or sublicensee under the agreement.

Either party may terminate
the agreement in the event of a material breach by the other party, subject to prior notice and the opportunity to cure. Sanofi may terminate the agreement upon 90 days prior written notice, however doing so will not relieve Sanofi of its
obligations to pay royalties with respect to further sales of any licensed product candidate.

Our discovery programs are focused on engineering new compounds and classes of compounds to treat the most highly resistant pathogens. In 2011, we initiated preclinical programs RX-05, focused on the development of
a novel scaffold for the treatment of serious multi-drug resistant bacterial infections, and RX-06, focused on the development of a novel scaffold for the treatment of serious fungal infections. The current goals for the RX-05 and RX-06 programs are
to show proof-of-concept by designing, preparing and validating at least one novel chemical scaffold that meets the following criteria:



binds in the intended region as measured crystallographically;



has functional activity at the target; and



shows activity in cell culture.

We expect that achievement of this proof-of-concept will provide the necessary chemical foundation for lead identification/lead optimization
programs in 2012. In addition, our RX-02 novel macrolide program is designed to overcome known ribosomal resistance modifications in a wide range of pathogens, including those generally associated with hospital-acquired Gram-positive infections,
community respiratory tract infections and skin infections seen both in hospital and community settings. RX-02 is a discovery program that we are not currently developing on our own and for which we are currently seeking partners.

Our Proprietary Drug Discovery Platform

Our
integrated approach to novel antibiotic design targets the large (50S) ribosomal subunit of bacteria, for which our co-founder, Yale Professor Thomas A. Steitz, Ph.D., shared the Nobel Prize in Chemistry in 2009. We believe our key competitive
advantage is our focus on the three-dimensional properties of antibiotics, which is enabled by our proprietary drug discovery platform. Unlike traditional approaches to antibiotic discovery, which generally rely on random screening of chemical
libraries to identify potential compounds, our discovery team combines sophisticated and exclusive computational tools with patent-protected, atomic-level insights into the structure of the ribosome to systematically engineer novel antibiotics to
avoid resistance and optimize potency, spectrum, efficacy and safety. As a result, we have created a highly efficient and productive drug development engine based on our unique design strategy that effectively leverages structure-based drug design,
preparative medicinal chemistry, ribosome biochemistry, molecular biology and pharmacology.

Because its protein building function is
essential for the life of the bacterium, the bacterial ribosome is the target of most marketed antibiotics, which work by binding to the ribosome and inhibiting its function. Unlike typical targets for structure-based drug design, the ribosome
presents numerous distinct targets for drug discovery. We believe that we are the only company with the combined intellectual property portfolio, cross-functional experience and knowledge base to exploit the high-resolution X-ray crystal structures
of the bacterial ribosome for drug development. By applying our analytic capability to current antibiotic classes, we are able to reveal gaps in coverage and rationally design next-generation, expanded-spectrum compounds and novel classes of
antibiotics to combat known resistance mechanisms.

Our ribosome crystallography tools relevant to our current development programs
consist of the proprietary experience, knowledge and data we have collected from X-ray crystallographic analysis of the structure of the ribosome and nearly 400 antibiotics which bind to the 50S subunit of the ribosome of the Haloarcula
marismortui bacteria, 25 antibiotics which

bind to the 50S subunit of the ribosome of another bacteria related to Enterococci and Staphylococci bacteria, and 12 antibiotics which bind to the 70S subunit of the Thermus
thermophilus bacteria. These tools offer four key competitive advantages, based on atomic-level detail of the structure of the ribosome, for the design of new anti-infectives with differentiated coverage, safety and convenience:



unrivaled three-dimensional knowledge of how anti-infectives interact with the ribosome;



ability to augment and refine this knowledge by characterizing new antibiotic structures on a regular basis;



insight into the location of resistance-causing ribosomal mutations; and



ability to utilize ribosomal spaces to fine tune molecular attributes important for broad-spectrum efficacy and safety.

Our computational design tools, which were built and optimized on sophisticated software from Professor William Jorgensen, Ph.D., at Yale
University, allow us to:



rapidly identify the high value binding sites for building new anti-infectives;



assess the impact of and design molecules to counter target-based resistance; and



query and capitalize on the molecular properties that drive permeability, efflux avoidance and pharmaceutical viability.

We believe that a fundamental strength of our product platform is our ability to proceed from a new hypothesis to generating a set of chemically
and biologically validated compounds in less than two weeks. This permits rapid pattern recognition, hypothesis refinement and iterative design. We can begin in any one of the validated ribosome target spaces, designing virtual molecules that extend
from a simple scaffold that binds well to the ribosome to nearby spaces that address a particular therapeutic objective, as the biology dictates. Representative subsets of these virtual molecules are then chemically synthesized with key building
blocks that can be assembled rapidly and on a scale that will facilitate fast and broad profiling, including measurements of affinity, microbiological activity, in vitro safety and in vivo efficacy in mouse models of infection. This
strategy has led, and we believe will continue to lead, to a pipeline of novel, small-molecule antibiotic classes, which not only will offer new therapies for emerging medical needs but have the potential to keep one step ahead of resistance.

Commercialization Strategy

Our
commercialization strategy is to develop our product candidates into leading therapies that will be available worldwide for the treatment of serious, multi-drug resistant infections. We have retained worldwide commercial rights to all of our product
candidates, except in the RX-04 program where we have retained an option for U.S. co-commercialization rights on a compound of our choosing. We intend to retain significant control over the commercial execution of each our product candidates, while
participating in a meaningful way in the economics of all drugs that we bring to market.

We intend to commercialize our product
candidates in the United States alone or in collaboration with one or more pharmaceutical companies that have established commercial capabilities, as appropriate, to maximize the value of each our product candidates. We currently have limited
marketing, no sales nor distribution capabilities. We intend to build a commercial organization in the United States to focus on educating hospital and institution-based physicians, nurses, pharmacy directors, and payors about our products. We
intend to recruit

experienced marketing, sales and medical education professionals and to develop a commercial strategy to target institutions with the greatest use of drugs for resistant, serious infections.

We will also seek to market our product candidates in the European Union, which has similarly seen an increase in serious, resistant
infections such as MRSA. We have performed an analysis of the European Union market for serious, resistant infection treatment, and our current plan is to commercialize our product candidates in the European Union with the assistance of partners.

We also believe that there is a rapidly growing need for antibiotics to treat serious, resistant infections in other markets throughout
the world, including Asian markets such as Japan, Korea, and Taiwan and emerging markets such as China, Russia, South America and India. We envision expansion of our product candidates to these markets through partnerships following the
introductions in the United States.

Our commercialization strategy with respect to specific product candidates is as follows:

Delafloxacin. We are currently developing our most advanced product candidate, delafloxacin, for the treatment of ABSSSI.
Assuming the successful completion of clinical trials and receipt of regulatory approvals, we intend to expand the usage of delafloxacin into other indications, such as CABP and cIAI. We plan to file for European Union approval of delafloxacin in
the same time period as we seek approval in the United States. Upon any such approval, we intend to focus our delafloxacin commercialization efforts on use primarily in hospitals. We believe that delafloxacin has the potential to become the
first-line treatment of choice in these settings because of its safety profile and coverage of both Gram-negative infections and Gram-positive infections, including MRSA.

Radezolid. Subject to obtaining sufficient additional funding beyond the proceeds of this contemplated offering, we intend to pursue development of radezolid for the treatment of ABSSSI, and intend to
conduct a Phase 2 trial prior to designing and initiating Phase 3 development in this indication. We intend to perform additional clinical trials of radezolid for treatment of serious infections, such as ABSSSI and severe CABP, and for long-term
treatment of underserved serious infections, such as osteomyelitis and prosthetic and joint infections, including as a result of orthopedic surgery. We believe that radezolid has the potential to become the antibiotic of choice for these indications
because of its potency and improved long-term safety profile.

RX04 Program.Under our collaboration and license
agreement with Sanofi, we have the right to co-commercialize in the United States a product of our choice developed through our RX-04 program. We are not required to exercise this option until six months prior to the commencement by Sanofi of Phase
3 trials of the product candidate. This timing allows us to review Phase 2 data on the efficacy and safety profiles of the product candidates prior to our selection, thus potentially lowering the commercial risk of exercising that option. We believe
that a single Rib-X sales force will be able to promote sales of delafloxacin, radezolid and an RX-04 product candidate, thus lowering our overall commercialization costs.

Manufacturing

We do not own or operate manufacturing facilities for the production of any of
our product candidates, nor do we have plans to develop our own manufacturing operations in the foreseeable future. All of our product candidates are organic compounds of low molecular weight, commonly referred to as small molecules. They are
manufactured in simple synthetic processes from readily available starting materials. We currently rely on a small number of third-party

contract manufacturers for all of our required raw materials, drug substance and finished product for our preclinical research and clinical trials. We do not have long-term agreements with any of
these third parties. We also do not have any current contractual relationships for the manufacture of commercial supplies of any of our product candidates after they are approved. If any of our products are approved by any regulatory agency, we
intend to enter into agreements with third-party contract manufacturers for the commercial production of those products. We currently employ internal resources to manage our manufacturing contractors.

Intellectual Property

The proprietary
nature of, and protection for, our proprietary drug discovery platform, our product candidates and our discovery programs, processes and know-how are important to our business. We seek patent protection in the United States and internationally for
our proprietary drug discovery platform, delafloxacin, radezolid, the RX-04 program and our discovery programs, and any other technology to which we have rights, where available and when appropriate. Our policy is to pursue, maintain and defend
patent rights, whether developed internally or licensed from third parties, and to protect the technology, inventions and improvements that are commercially important to the development of our business. We also rely on trade secrets that may be
important to the development of our business.

Our commercial success will depend in part on obtaining and maintaining patent protection
and trade secret protection of our proprietary drug discovery platform, our current and future product candidates and the methods used to develop and manufacture them, as well as successfully defending these patents against third-party challenges.
Our ability to stop third parties from making, using, selling, offering to sell or importing our products and technology depends on the extent to which we have rights under valid and enforceable patents or trade secrets that cover these activities.
We cannot be sure that patents will be granted with respect to any of our pending patent applications or with respect to any patent applications filed by us in the future, nor can we be sure that any of our existing patents or any patents that may
be granted to us in the future will be commercially useful in protecting our technology. For this and more comprehensive risks related to our intellectual property, please see Risk FactorsRisks Relating to Our Intellectual
Property.

Our Proprietary Drug Discovery Platform

As of the date of this prospectus, we have an exclusive license from Yale University under which we possess rights to certain patents, patent applications and other intellectual property related to the high
resolution X-ray crystal structure of the 50S ribosome from Haloarcula marismortuii, as well as additional technology related to 50S ribosome structures from Haloarcula marismortuii mutants. In addition, under our license from Yale, we
have further rights related to 70S ribosome structures from Thermus thermophilus. We have developed additional technology both jointly with Yale and independently. We have also exclusively licensed 30S ribosome technology from the Medical
Research Council.

The patent portfolio for our proprietary drug discovery platform is directed to drug discovery methods, ribosome
crystal forms and methods of making them. This portfolio includes issued U.S. patents (U.S. Pat. No. 7,666,849, U.S. Pat. No. 7,606,670, U.S. Pat. No. 7,504,486, U.S. Pat. No. 7,079,956, U.S. Pat. No. 6,952,650, U.S. Pat. No. 6,947,845, U.S.
Pat. No. 6,947,844, U.S. Pat. No. 6,939,848, U.S. Pat. No. 6,925,394, and U.S. Pat. No. 6,638,908), pending U.S. patent applications, and corresponding issued and foreign national or regional counterpart patents or applications. The issued patents,
if the appropriate maintenance, renewal, annuity or other governmental fees are paid, are expected to expire between 2021 and 2022.

As of the date of this prospectus, we have a license, both exclusive and nonexclusive, from Wakunaga Pharmaceutical Company, Ltd. to certain patents and patent applications and to certain patents and patent
applications to Abbott Laboratories. We have also licensed further technology from CyDex Pharmaceuticals, Inc. (now a wholly owned subsidiary of Ligand Pharmaceuticals Incorporated, both hereafter referred to as Ligand), for the use of Captisol, a
sulfobutyl ether beta-cyclodextrin excipient, in connection with delafloxacin. We have developed additional technology independently.

The patent portfolio for delafloxacin and delafloxacin meglumine, the active pharmaceutical ingredient in the delafloxacin product candidate, is
directed to composition of matter, formulation, manufacturing methods and methods of use. It includes issued U.S. patents (U.S. Pat. No. 6,133,284, U.S. Pat. No. 6,156,903, U.S. Pat. No. 5,998,436, U.S. Pat. No. 7,576,216, and U.S. Pat. No.
7,728,143), pending U.S. patent applications, and corresponding issued and foreign national or regional counterpart patents or applications. The issued composition of matter patents to delafloxacin generally, if the appropriate maintenance, renewal,
annuity or other governmental fees are paid, are expected to expire in 2016, excluding any additional term for patent term adjustments or patent term extensions. For delafloxacin meglumine, we have an issued U.S. patent (U.S. Pat.
No. 7,728,143) and two pending U.S. patent applications (U.S. Serial No. 12/701,254 and U.S. Serial No. 12/763,476). A Notice of Allowance has been issued (mailed April 23, 2012) by the USPTO for the pending U.S. Serial No.
12/701,254. If the appropriate maintenance, renewal, annuity or other governmental fees are paid, U.S. Pat. No. 7,728,143, is expected to expire in 2027 and the two pending U.S. patent applications, if issued, are expected to expire no earlier
than 2025. We believe that additional term for one of our patents for delafloxacin or delafloxacin meglumine of up to five years may result from the patent term extension provisions of the Hatch-Waxman Amendments of 1984, or Hatch-Waxman. We expect
that the other patents, and patent applications in the portfolio, if issued, and if the appropriate maintenance, renewal, annuity or other governmental fees are paid, would expire between 2025 and 2029.

Radezolid

The patent portfolio for
radezolid is directed to cover composition of matter, formulation, manufacturing methods and methods of use. It includes issued U.S. patents (U.S. Pat. No. 6,969,726, U.S. Pat. No. 7,148,219, U.S. Pat. No. 7,456,206, U.S. Pat No.
7,705,026), pending U.S. patent applications, and corresponding issued and foreign national or regional counterpart patents or applications. We expect the composition of matter patent, if issued, and if the appropriate maintenance, renewal, annuity
or other governmental fees are paid, to expire in 2024, excluding any additional term for patent term adjustments or patent term extensions. We believe that additional term for one of our radezolid patents of up to five years may result from the
patent term extension provisions of Hatch-Waxman. We expect the other patents and patent applications in the portfolio, if issued, and if the appropriate maintenance, renewal, annuity or other governmental fees are paid, to expire between 2024 and
2031.

RX-04 Program

The patent
portfolio for our RX-04 program is directed to cover composition of matter and methods of use. It includes pending PCT Patent Applications (PCT Pub. No. WO 2011/047319, PCT Pub. No. WO 2011/047320, and PCT Pub. No. WO 2011/047323) and other pending
provisional U.S. patent applications and corresponding foreign patent applications. If issued, and if the appropriate maintenance, renewal, annuity or other governmental fees are paid, we expect that these would have terms that would extend at least
until 2030, excluding any

additional term for patent term adjustments or patent term extensions. We believe that additional exclusivity for composition of matter patents of up to five years, may result from the patent
term extension provisions of Hatch-Waxman.

In
addition to patents, we rely on trade secrets and know-how to develop and maintain our competitive position. For example, significant aspects of our proprietary drug discovery platform are based on unpatented trade secrets and know-how. Trade
secrets and know-how can be difficult to protect. We seek to protect our proprietary technology and processes, in part, by confidentiality agreements and invention assignment agreements with our employees, consultants, scientific advisors,
contractors and commercial partners. These agreements are designed to protect our proprietary information and, in the case of the invention assignment agreements, to grant us ownership of technologies that are developed through a relationship with a
third party. We also seek to preserve the integrity and confidentiality of our data, trade secrets and know-how by maintaining physical security of our premises and physical and electronic security of our information technology systems.

Trademarks

Further, we seek trademark
protection in the United States where available and when appropriate. We have filed for trademark protection in the United States for the RIB-X mark and the RIB-X PHARMACEUTICALS ANTIBIOTICS IN THREE DIMENSIONS mark, which we use in connection with
our pharmaceutical research and development as well as our product candidates. The RIB-X mark has been approved for publication by the USPTO, but is subject to a 30-day public opposition period, which can be extended by an additional 90 days upon
the request of an interested party.

Other Commercial Agreements

Wakunaga Pharmaceutical Company License Agreement

On May 12, 2006, we entered into a
license agreement with Wakunaga Pharmaceutical Company, Ltd. under which we acquired rights to certain patents, patent applications, and other intellectual property related to delafloxacin. Under the license, we are responsible for and must use
commercially reasonable efforts in conducting all research, pre-clinical and clinical studies, and other development and commercialization activities for the licensed compound and licensed products. We also have exclusive control over and
responsibility for the regulatory strategies relating to the development and commercialization of the licensed compound and licensed products. We have the right, in our sole discretion, to institute, prosecute and control any action or proceeding to
restrain infringement of the licensed patents and we are responsible for defending and controlling any action or proceeding with respect to patent infringement involving our use, sale, license or marketing of the licensed products. Under the license
we also have the right to grant sublicenses.

Wakunaga has certain termination rights, should we fail to perform our obligations under the
agreement, if we become subject to bankruptcy or similar events, or if our business is transferred or sold and the successor requires us to terminate a substantial part of our development activities under the agreement. We have the right to
terminate the license for cause upon six months written notice to Wakunaga. Unless earlier terminated, the license agreement will continue in effect on a country-by-country and product-by-product basis until we are no longer required to pay any
royalties, which is the later of the date the manufacture, use or sale of a licensed product in a country is no longer covered by a valid patent claim, or 15 years following the first commercial sale in such country. The last issued patent
licensed under the agreement to expire in the United States, assuming no patent term extensions under Hatch-Waxman are granted, will be U.S. Pat. No. 7,728,143, which expires in 2027.

We paid Wakunaga $1.5 million upon execution of the license. In June 2007 and September 2009, we made milestone payments under the agreement of
$2.0 million and $1.5 million, respectively. In addition, we may be required to pay to Wakunaga an aggregate of $15.0 million upon the achievement of specified development and regulatory approval milestones. We are also obligated to pay tiered
royalties in the single digits on net sales of licensed products. We are also obligated to pay a substantial portion of non-royalty income received in consideration of a sublicense of the Wakunaga technology. Through March 31, 2012, we have made
payments under the license agreement totaling $5.0 million.

Yale University License Agreement

On December 6, 2001, we entered into an exclusive license agreement with Yale University under which we acquired rights to certain patent
applications and other intellectual property. We subsequently entered into three amendments to this license agreement to update the license agreement to include patent applications filed after the effective date of the original license
agreement and to exclusively license additional technology from Yale. We are obligated to meet certain diligence requirements, including designing a plan for developing and commercializing the licensed products, using reasonable commercial
efforts to begin implementation of the plan, providing annually an updated plan to Yale and meeting other diligence milestones. We also have the right to grant sublicenses of the licensed rights to third parties. We are primarily responsible for the
preparation, filing, prosecution and maintenance of all patent applications and patents covering the licensed intellectual property. We have the first right, but not the obligation, to enforce the licensed intellectual property against infringement
by third parties and to conduct the legal defense of any third party claims alleging patent infringement against us with respect to the licensed products and intellectual property.

Upon the occurrence of certain events, Yale has the right to terminate the license agreement upon 60 days written notice to us, should we fail to
make a material payment under the agreement, commit a material breach of the agreement, fail to carry insurance required by the agreement, cease to carry on our business or become subject to bankruptcy or similar insolvency event. We have the right
to terminate the license agreement upon 90 days written notice to Yale. Unless earlier terminated, the agreement will continue in effect until the last of the licensed patents expires. The last issued patent licensed under the agreement to expire in
the United States, assuming no patent term extensions under Hatch-Waxman are granted, will be U.S. Pat. No. 7,504,486, which expires in 2022.

Under the agreement, we are required to make certain payments of up to $900,000 to Yale upon the achievement of specified development and regulatory approval milestones for each of the first three products
developed under the license. We are also obligated to pay royalties in the single digits on net sales of licensed products or services. Through March 31, 2012, we have